I use Mobil1 15W-50 even in the winter. It has the same pour point as 5W-40 and from my freezer experiment they don't flow much differently at freezer temps, the 5W-40 is just a little more fluid but not much. Hatterasguy is right about the 0W-40, it's not for our old diesel engines unless you live beyond the polar circle. I tried it briefly in my 82 300D and the engine didn't like it after a cold start. Lots of shaking and blue smoke.

__________________
2004 VW Jetta TDI (manual)

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