I use shell Rotella synthetic 5w40 in my benz all year. Even though its not a "true synthetic" It is the best value for my 300k mile 25 year old daily driver. Its better than regular dino at least. I have thought about using the same oil that I put in my duramax and TDI beetle which is the valvoline 5w40 full synthetic blue extreme. It meets all major diesel engine requirements and is one of the only oils endorsed by cummins, catipillar, volvo, onan, detroit. The bottle says it meets a mercedes spec but i can't remember which one. I dunno what anyones thoughts are on that. It seems to be great oil as at 5000 miles it still comes out pretty clean and I can see no build up or varnishing inside the engine. It also got rid of the valve ticking on my buddies gas chev engine. Only problem is it can be hard to find!

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1987 300D 180k
1985 300SD 330k Helga
1992 300D Lola 269k- Sold
1986 325 coupe Bessie >300k Original engine(ODO Broke) sold
2006 2500HD Druamax Linda- Tows boat and looks pretty in garage thats about it