Since I've heard that these engines could last an eternity if properly maintained I have a question: from your experience so far, what type of oil could keep a Mercedes Diesel engine most happy?

Who has the most miles with one type of engine oil?

Since now I've owned a 300TD with a 671.912 Diesel running on Castrol Protection and 290.000Km (180.197 miles) on the clock and a 300 SDL with 200.000 miles on the clock and Liqui Moly religiously changed at 2000-3000 miles intervals. Both engines run well and burn no oil whatsoever.

The previous owner of my 300SD claimed that the car has been running on Mobil's Delvac 1300 (non-synthetic) since new, which means for around 237,000 miles. I find, however, that it smokes less with "synthetic" Rotella 5w40, to which I recently switched.