I used Chevron Delo 400 15w-40 the last change. This is a great diesel oil that is also highly recommended on this board for gas engines. After I changed, I found out that Amsoil sells a synthetic 10w-40, which is what I think I'll use from now on since it will cover me for cold winter temperatures too. Don't use a 30 weight. The previous owner of my car had Amsoil 10w-30 in there, and I thought I had serious valvetrain noise. After changing to the (MB recommended) 40 weight, all that clicking noise went away. The synthetic 10w-30 weight was just too thin I think.
'84 300D, 173k
'90 300CE, 163k