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.