I recently switched to synthetic from regular 15W-40 in my 82 300D. For ease of starting and low startup wear I decided to first try 0W-40. This was about a month ago. I noticed instantly that the car idled rougher when cold. Well I lived with it for over a month until I decided to try the thicker 15W-50 oil and dump $50 worth of new oil. Boy what a difference! Now I have a smooth cold idle again. My oil consumption was low even on the thinner oil, but I think what's happening here is that the thicker oil naturally dampens any vibration that a cold diesel engine may be producing.

I also started using the 15W-50 in my 87 300D and noticed like others were saying on this board that the engine runs quieter, at least when it's warmed up. This oil is a miracle!

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL