quaker state FCI branded 15W40 is the only oil for this car my indy mechanic recommends based on him telling me that's what the mercedes dealerships use....however i use castrol 15W40 and oddly enough, while oil is oil, my car starts up smoother in the mornings when the oil gets a little older compared to Q.S...also it warms up and gets out of higher idle faster...but that concerns me aswell, perhaps castrol doesn't handle heat aswell and casues the faster warm up? I'm surprised for all the money that goes into research and consumer protection by for profit private companies that there's no real indepth engine oil analysis in regards to all the different brands out there.

then again, Shell motor oil, quaker state oil, penzoil ....they're all the same company.....