In my 84 300td at around 20 degrees I use the block heater and I use a mix of Rotella 15-40 and Mobil 1 0-40w,(50-50) car uses a quart about every 800 miles using this mix. using straight mobil 1 I use a qt about every 500 miles. leaks.

Other diesels I use Mil Spec 2104C surplus If I can get it. One of the highest TBN's available. It either comes 10w or 10w-30. It's a bluish color. Interesting stuff.