Just my $.02...

I used 10W-40 until I passed the 400K mark and was smoking like a freight train...

I then switched to 20W-50 (actually my mechanic suggested it) and the smoking stopped. With the miles I have on my 240D, I use this grade year-round with no problems.

I have even began using it in the wife's 300D and the car runs great!

Like I said... My $.02 worth....

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William
76 240D - 545K miles
78 300D - 200K+ miles