For what it is worth I have been using M1 0W-40 in my 300CE (M103) for the last couple oil changes. I have no leaks to speak of. I did have to replace the timing cover gasket and valve cover gasket before last change though they were leaking before switching and we all know these items can be a common source of leaks on any older M103. Once resealed all is good. 15W-40 is a bit too cold to use here in the winter when temps go below freezing. Rotella T6 is a good oil but quite pricey up here. I cross the border to get 0W-40 since it is half the price and easily found at any Walmart. Cold temperatures, stop and go along with short drives makes me think a synthetic oil is a good choice for my driving conditions. I think we can all agree to getting a bit carried away with oil recommendations. Oil change intervals and quality of your oil filter are a lot more important to engine life than what particular oil you use...

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1989 300CE