Today while at the local Pep Boys, I discovered a new type of oil especially formulated for small truck and car diesel engines. It's Castrol RX 15W-40 Heavy Duty. I picked up a case for my next oil change. It says on the back of the bottle that it meets Mercedes-Benz and VW diesel engine oil standards. I generally use Castrol GTX 10W-40 year-round, but will give this a try and let you all know how it goes. Also, it was the same price as regular Castrol GTX ($1.39/quart).
'67 250SE Cabriolet
'93 300E 2.8