I recently bought an '85 300D with a history of Mobil 1 as its engine lubricant, which I have continued. I know that one advantage of synthetic oil is supposed to be extended oil change intervals, but I'd like your collective advice on how long that interval should be. On our Euro 300D, in which I use Delo 400, I change the oil every 3000 miles.

The '85 has about 250,000 miles on its original engine, which is in outstanding condition. How often should I change the Mobil 1 to keep the engine that way?

Phil
'85 300D, California model
'84 Euro 300D, peasant class/manual trans