5w30 vs 5w40

[Breeze1]

To my surprise there seems to be an eternal debate about which one is better.

In my opinion it's a no brainer since the number at the left of the W indicates protection in cold conditions (the lower the number, the thinner the oil is) and the one at the right the ability of oil particles to sustain high temps whithout breaking down i.e. hot conditions protection.

So for e.g. I would always go for castrol edge 5w40 instead of 5w30. The only thing that would make me go for a "30" would be if the number on the left was higher, since engines are most vulnerable to friction on cold starts.

Any inputs?

[02Pilot]

Oil is like religion - best not publicly discussed except in the most unambiguous, objective terms unless you want to start an argument that no one can win.

Your engine calls for LL-01, which is functionally very similar to ACEA A3/B4 (go to the ACEA site and you can see all the measurements that make up the latter spec). Oil weight is only one of a number of important criteria to consider. You cannot say that deviating from the called-for spec will necessarily have negative consequences, at least not without a very broad data set.

Weight-in-grade varies from oil to oil. Some 5wX oils will be lighter at 40C than others. What is your primary concern? Minimizing cold start-up wear? If so, then this should be a consideration (if you want to delve that deeply into it). If, on the other hand, you are more concerned with other factors (sustained high RPM use, longevity, minimizing frictional losses, etc.), then there are other specifications that are more important.

Do your research and make your choice. Without degrees in tribology and mechanical engineering, that's all you can do.

[325bob]

To my surprise there seems to be an eternal debate about which one is better.

In my opinion it's a no brainer since the number at the left of the W indicates protection in cold conditions (the lower the number, the thinner the oil is) and the one at the right the ability of oil particles to sustain high temps whithout breaking down i.e. hot conditions protection.

So for e.g. I would always go for castrol edge 5w40 instead of 5w30. The only thing that would make me go for a "30" would be if the number on the left was higher, since engines are most vulnerable to friction on cold starts.

Any inputs?

Yes, its that simple, thicker is better protection when hot, the "multi-weight" misnomer has caused decades of arguments, basic physics teaches that most liquids get thinner when heated,and 40w at 32F is prolly a 10 w at250F, but it can't be 40 and 10 at once.

[Breeze1]

Thanks for your informative inputs. I've personally always followed the "full synthetic" rule and chose grades as described above, as well as closer than specified oil changes intervals. Used to put Liquimoly Top Tech 5w40 in my previous engine, switched to castrol edge titanium 5w40 with the current one (for no particular reason).

I live in a warm country so the "number at the left" (for lack of better words) doesn't matter that much, the one on the right does, as I like to make a few pulls to redline here and there (always on a warm engine with oil pressure no higher than 18psi at idle). But usually it doesn't get higher than "40" unless going for a 10w60 which is much more expensive and probably not necessary since I don't race my car, and it was meant for higher revving M engines as far as I know.

[02Pilot]

Read the relevant portions of the following and consider the implications of what these standards actually mean. You will realize that there is much, much more to this than oil grade: https://www.acea.be/uploads/news_doc...ugust_2018.pdf

[bmwdirtracer]

Pick an oil you like. Don't tell anyone what it is.

Now, as for calling an oil "Titanium"? What's that smell?

[moroza]

Oil is probably the biggest can of worms in the automotive world, and lacking a degree in tribology I don't want to touch it except for what I do know...

In my opinion it's a no brainer since the number at the left of the W indicates protection in cold conditions (the lower the number, the thinner the oil is) and the one at the right the ability of oil particles to sustain high temps whithout breaking down i.e. hot conditions protection.

The numbers are viscosity indices. Viscosity is a major component of but not the same as protection (including but not limited to lubrication).

Your engine calls for LL-01, which is functionally very similar to ACEA A3/B4 (go to the ACEA site and you can see all the measurements that make up the latter spec). Oil weight is only one of a number of important criteria to consider.

True. Otherwise we'd be using maple syrup for 0W20 and molasses for 20W50.

Weight-in-grade varies from oil to oil. Some 5wX oils will be lighter at 40C than others.

True. Furthermore, a 5W40 (especially a non-synthetic) can often be thicker at cold temps than a 5W30, even though their "cold" weights are the same. (EDIT: I had 30 and 40 backwards)

Yes, its that simple, thicker is better protection when hot

False. Heat up some bearing grease, pour it into your engine, and see how that works out. An engine designed for 0W20 will not be better-protected at high temperatures if filled with 10W60.

Thanks for your informative inputs. I've personally always followed the "full synthetic" rule and chose grades as described above, as well as closer than specified oil changes intervals. Used to put Liquimoly Top Tech 5w40 in my previous engine, switched to castrol edge titanium 5w40 with the current one (for no particular reason).

There seems to be no argument that full-synthetic oil is superior to conventional (with the caveat that it can remove deposits that were keeping oil from leaking), but more frequent changes are not always better. For example, on Volkswagen TDI, it's a fairly well-documented issue that changing oil too often leads to increased wear, because detergent additives that correct oil comes with get consumed throughout the normal OCI, and if replenished too often will essentially degrease engine internals.

I live in a warm country so the "number at the left" (for lack of better words) doesn't matter that much

Ambient temps do factor in, but an engine that hasn't run in a few hours is cold - compared to its normal operating temperature of ~375K - regardless of whether it's 225K or 325K outside. It experiences much more wear at startup than when up to temp, no matter what weight oil is used. Part of the increased wear is due to oil draining down into the sump, which tends to be worse with thin oil.

I like to make a few pulls to redline here and there (always on a warm engine with oil pressure no higher than 18psi at idle). But usually it doesn't get higher than "40" unless going for a 10w60 which is much more expensive and probably not necessary since I don't race my car, and it was meant for higher revving M engines as far as I know.

The decisive factor is not how hard you drive it per se, but how high oil temperature gets. If your oil gets heated significantly beyond what the engine designers had in mind, you may indeed benefit from higher weight. That takes situations like racing, not just redlines here and there. M cars that call for 10W60 are designed to withstand actual Motorsports use. You have a (real) pressure gauge? How about a temperature gauge?

Viscosity is a tradeoff between pressure and flow, and different engines are designed for different sweet spots in that tradeoff. Oil that's too thin may not have enough pressure to keep metal from metal. Oil that's too thick may not have enough flow to absorb heat - potentially leading to chemical breakdown and other problems even if its viscosity and pressure are still ok - or get to everywhere it needs to be (the valvetrain, for example). Thick oil places increased stress on the oilpump and anything that drives it. For example, some Ford engines are prone to shearing a shared distributor/oilpump drive gear pin if oil is too thick. Other physical properties held constant (Lucas stabilizer is a well-known additive that counteracts this), thin oil also tends to stick to parts less effectively, and not be around for the next startup until pumped.

I'll add a datapoint: my E34 544iT picked up ~8% fuel economy when I switched from Amsoil 5W40 to 5W30.

[bmwdirtracer]

I think molasses is a straight 90 weight, and grade A Vermont maple syrup is a 10/40, but tastes great.

[moroza]

There's increasing consumer awareness of the dangers of synthetic syrup, even though it's cheaper. Pringles had a thing back in the late 90's where certain additives caused rear main seal leakage (look up Olestra); definitely a case where conventional stuff is safer! And then there's France, which per some EU directive recently had to adjust their syrup specs to better deal with ash content from one third of their traditional three-C breakfast (the other two being Coffee and Croissant).

[Breeze1]

Wow, some major knowledge shared in here, including on maple syrup and molasses.
Chris: the "Titanium FST" formula is *supposed to* have added benefits regarding film strength, over the regular "Edge" (pun not intended).

https://oilgroupbg.com/pdf/castrol/c...m-fst-5w40.pdf

Moroza: Oil pressure gauge and oil temp gauge, yes. The reason I tend to rely on oil pressure more than oil temp to know when I can rev the engine freely is that, while there is an obvious correlation between oil temp going up and pressure going down in the city, it's not that obvious at road speeds (especially highway) since the oil temp can get as low as 65°-70°C even after hours of driving. Usually once the pressure drops to 16-18Psi at idle it stays there no matter what the oil temp is, but only usually. If I have been driving at 2000-2500rpm for 30min for example, I would put the car in neutral for a few secs and verify that the pressure at idle hasn't risen before revving it. I know it's not necessary but I do it for good measure.

[pbonsalb]

I use Mobil 1 0W40 in an E61 535xi that specifies 5W30 and in an E90M3 that specifies 10W60 and in a fully built motor E36M3 for which the engine builder specified any synthetic oil that is not too thin or thick for the driving use.

[325bob]

Oil is probably the biggest can of worms in the automotive world, and lacking a degree in tribology I don't want to touch it except for what I do know...



The numbers are viscosity indices. Viscosity is a major component of but not the same as protection (including but not limited to lubrication).



True. Otherwise we'd be using maple syrup for 0W20 and molasses for 20W50.



True. Furthermore, a 5W30 (especially a non-synthetic) can often be thicker at cold temps than a 5W40, even though their "cold" weights are the same.



False. Heat up some bearing grease, pour it into your engine, and see how that works out. An engine designed for 0W20 will not be better-protected at high temperatures if filled with 10W60.



There seems to be no argument that full-synthetic oil is superior to conventional (with the caveat that it can remove deposits that were keeping oil from leaking), but more frequent changes are not always better. For example, on Volkswagen TDI, it's a fairly well-documented issue that changing oil too often leads to increased wear, because detergent additives that correct oil comes with get consumed throughout the normal OCI, and if replenished too often will essentially degrease engine internals.



Ambient temps do factor in, but an engine that hasn't run in a few hours is cold - compared to its normal operating temperature of ~375K - regardless of whether it's 225K or 325K outside. It experiences much more wear at startup than when up to temp, no matter what weight oil is used. Part of the increased wear is due to oil draining down into the sump, which tends to be worse with thin oil.



The decisive factor is not how hard you drive it per se, but how high oil temperature gets. If your oil gets heated significantly beyond what the engine designers had in mind, you may indeed benefit from higher weight. That takes situations like racing, not just redlines here and there. M cars that call for 10W60 are designed to withstand actual Motorsports use. You have a (real) pressure gauge? How about a temperature gauge?

Viscosity is a tradeoff between pressure and flow, and different engines are designed for different sweet spots in that tradeoff. Oil that's too thin may not have enough pressure to keep metal from metal. Oil that's too thick may not have enough flow to absorb heat - potentially leading to chemical breakdown and other problems even if its viscosity and pressure are still ok - or get to everywhere it needs to be (the valvetrain, for example). Thick oil places increased stress on the oilpump and anything that drives it. For example, some Ford engines are prone to shearing a shared distributor/oilpump drive gear pin if oil is too thick. Other physical properties held constant (Lucas stabilizer is a well-known additive that counteracts this), thin oil also tends to stick to parts less effectively, and not be around for the next startup until pumped.

I'll add a datapoint: my E34 544iT picked up ~8% fuel economy when I switched from Amsoil 5W40 to 5W30.

And the EPA would like you to use 0W0 for CAFE compliance, but we'll all be in Teslas by then. You called my response false,but explained it well later?

[MarcoZandrini]

THIS! BMW specs 0w20 in the B58 (M240i, etc.) engine in the USA. The specd oil everywhere else in the world is 5W30. I run LiquiMoly 5W30 LL01FE in my M240i. I didn’t buy the car for its fuel economy!

[ross1]

There's increasing consumer awareness of the dangers of synthetic syrup, even though it's cheaper. Pringles had a thing back in the late 90's where certain additives caused rear main seal leakage (look up Olestra); definitely a case where conventional stuff is safer! And then there's France, which per some EU directive recently had to adjust their syrup specs to better deal with ash content from one third of their traditional three-C breakfast (the other two being Coffee and Croissant).

Never heard it described that way before, can't wait to use that one.
RE oil
I've been fixing cars for better than 40 years and have never, ever seen or heard of an engine failure because it had the "wrong" oil in it.

An oil burning Datsun I briefly owned ran merrily along on 50% STP, another old Chevy on a "proprietary mix" of waste oil and gear lube. Both soldiered on.
Use what your manufacturer specs or at least close, avoid gimmick additives and for Heaven's sake change it once in a while.
Over and out

[moroza]

And the EPA would like you to use 0W0 for CAFE compliance, but we'll all be in Teslas by then. You called my response false,but explained it well later?

I said it was false that it's that simple, that thicker is always better protection when hot. First, it's not that simple because there's more to oil than viscosity. Second, assuming we're not talking about extreme overheating (say, an engine designed to run with 95C oil actually running at 350C), oil that's the right viscosity at 350C will be too heavy at 95C, and will not protect the engine better. The example of bearing grease illustrates the "heavier is better" oversimplification taken to its logical conclusion.

Never heard it described that way before, can't wait to use that one.
RE oil
I've been fixing cars for better than 40 years and have never, ever seen or heard of an engine failure because it had the "wrong" oil in it.

Does oil that's never been changed count as "wrong"?

Outright failure? Yes, unlikely (but try pouring your Chevy's "proprietary blend" into an S54 and see how many redline pulls you can make). Premature wear of bearings (clearance out of spec by 100k instead of 250k), or lesser problems like oilpump damage (Ford 300/4.9 sixes, among others)? Certainly possible.

An oil burning Datsun I briefly owned ran merrily along on 50% STP, another old Chevy on a "proprietary mix" of waste oil and gear lube. Both soldiered on.
Use what your manufacturer specs or at least close, avoid gimmick additives and for Heaven's sake change it once in a while.
Over and out

Yikes. Agreed with your conclusion: change it on time, and use what's specified, more or less. 5W40 instead of 5W30 is practically just fine, and if you have specific reasons to use the heavier stuff, go for it.

Another data point: with Castrol Edge 5W30 (used twice), my 544 got lifter tick after ~10k miles if it sat for over a week, and the oil was a light-medium brown by then. With Amsoil (used at least three times), I had no such problems and the oil was still golden between 12 and 14k. No analysis done but I'd put money on the Amsoil being cleaner despite higher miles, and despite Castrol formally meeting BMW specs.

[moroza]

Moroza: Oil pressure gauge and oil temp gauge, yes. The reason I tend to rely on oil pressure more than oil temp to know when I can rev the engine freely is that, while there is an obvious correlation between oil temp going up and pressure going down in the city,

Hm? Oil temp would be higher with higher RPM, which is on freeways rather than city streets, no?

My understanding is that the reason to monitor oil temperature to know when to load the engine is because it's a better indication of the temperature of the bottom end. Coolant temp tells you how hot the cylinders and head are, but unlike oil, it doesn't flow to the rotating assembly. My rule of thumb has been to wait until the coolant is up to temp, and that same length of time again, before any WOT or over ~3500RPM.

[Breeze1]

Hm? Oil temp would be higher with higher RPM, which is on freeways rather than city streets, no?

My understanding is that the reason to monitor oil temperature to know when to load the engine is because it's a better indication of the temperature of the bottom end. Coolant temp tells you how hot the cylinders and head are, but unlike oil, it doesn't flow to the rotating assembly. My rule of thumb has been to wait until the coolant is up to temp, and that same length of time again, before any WOT or over ~3500RPM.

Not within speed limits, minus a few pulls here and there in my case. Within speed limits, most of the time, I just put it in 5th which equals somewhere around 2500/3000rpm (never really paid attention). On B roads, that's another story

You'd be surprised by how long it takes for the oil to reach its operating temp: at least 10 min, if not 15. I wait until the oil pressure settles because IMO that's the real indicator that it has reached its thinest state, which means it can flow everywhere in the engine. The temp varies according to the volume of air that flows through the car so for e.g. if you settle at 4000rpm in 3rd you'll have a temp of 80°C whereas the same 4000rpm in 4th will bring the temp down to 70° due to higher airflow.

Viscosity tends to be stable: once it has reached 16psi it stays there unless there are extended periods of low rpm driving in relatively cold conditions.

Anyway both methods work but I like the "definitive" aspect of the oil pressure level.

And there is nothing that drives the engine temps up like traffic jams, unless maybe proper racing. Because the engine is deprived from the -powerful- cooling effect of airflow. Even in water cooled engines the radiators are merely "amplifiers" of air cooling.

[bmwdirtracer]

Here are two excellent threads on oil, from M5 Board, where some serious tribology goes on. In the first thread listed ( a short, less complex thread), make sure to scroll down to where molybdenum is discussed. LiquiMoly has MoS2, which has been proven for over 40 years to be even more magnificent than ZDDP (which the EPA hates) I'm attaching a link to the MoS2 info too. The second thread has extensive documentation via lab reports from Blackstone and other labs.

https://www.m5board.com/vbulletin/e3...uggestion.html

https://www.m5board.com/vbulletin/e3...b-analyst.html


https://liqui-moly.com.au/what-is-mos2/


Please note: I am NOT suggesting that 10/60 oil is suitable for any car except those calling for it.

[325bob]

I said it was false that it's that simple, that thicker is always better protection when hot. First, it's not that simple because there's more to oil than viscosity. Second, assuming we're not talking about extreme overheating (say, an engine designed to run with 95C oil actually running at 350C), oil that's the right viscosity at 350C will be too heavy at 95C, and will not protect the engine better. The example of bearing grease illustrates the "heavier is better" oversimplification taken to its logical conclusion.



Does oil that's never been changed count as "wrong"?

Outright failure? Yes, unlikely (but try pouring your Chevy's "proprietary blend" into an S54 and see how many redline pulls you can make). Premature wear of bearings (clearance out of spec by 100k instead of 250k), or lesser problems like oilpump damage (Ford 300/4.9 sixes, among others)? Certainly possible.



Yikes. Agreed with your conclusion: change it on time, and use what's specified, more or less. 5W40 instead of 5W30 is practically just fine, and if you have specific reasons to use the heavier stuff, go for it.

Another data point: with Castrol Edge 5W30 (used twice), my 544 got lifter tick after ~10k miles if it sat for over a week, and the oil was a light-medium brown by then. With Amsoil (used at least three times), I had no such problems and the oil was still golden between 12 and 14k. No analysis done but I'd put money on the Amsoil being cleaner despite higher miles, and despite Castrol formally meeting BMW specs.

Semantics, I'll restate; too thin when hot is always worse!, theres a good reason the s54 needs 10w60. its known well as the "grenade".

[Breeze1]

Here are two excellent threads on oil, from M5 Board, where some serious tribology goes on. In the first thread listed ( a short, less complex thread), make sure to scroll down to where molybdenum is discussed. LiquiMoly has MoS2, which has been proven for over 40 years to be even more magnificent than ZDDP (which the EPA hates) I'm attaching a link to the MoS2 info too. The second thread has extensive documentation via lab reports from Blackstone and other labs.

https://www.m5board.com/vbulletin/e3...uggestion.html

https://www.m5board.com/vbulletin/e3...b-analyst.html


https://liqui-moly.com.au/what-is-mos2/


Please note: I am NOT suggesting that 10/60 oil is suitable for any car except those calling for it.

Great, thanks!