My guess on the issue you noticed: The engine produces heat that is not linear with speed since air resistance increases by the square of the speed....
Oil additives are bogus. Not that they don't do what they say they do, but whatever they say they do is not worth paying for. If they were necessary...
I have the same one. I have to put mine on the back to clear my subwoofer. TBH I can't really tell any difference unless the side windows are up then...