Transmission & Differential Fluid

[JSG]

I am preparing to change the subject fluids on my 2011 328i Coupe E90 with a 6 speed manual transmission. The physical chaging of the fluids seems rather simple but the selection of the fluids themselves is somewhat confusing. I read someplace that one should never use sythetic oil in the differential or the transmission. Since I have always been under the impression that sythetic is better I am wondering if this is true and, if so, why. Also, does anyone know the ideal specifications for the fluid themselves. I see that differential fluid can be 75W-90 or 75W-145. So for the differential I would need to know (a) the correct viscosity, (b) if there is an additional specification I need to look for, and (c) if the use of synthetic is advantageous? I am not certasin if I have a limited slip differential as I ordered the car new but did not specify it. Did they come standard in 2011? BTW, I did not order the sports/handeling package either.

As far as the transmission goes, and guidance there would be appreciated as well. I know the path of least resistance is probably to got to the dealer and buy the OEM fluids but I'm finding they are almost triple the price of all others on the market.

Thanks in advance for your help & guidance.