We got 300,000 miles out of a Chevy V6 (1990 or 1992) by changing the oil ever 3000 miles, regular 10W30 "dino".

I'd never use less than 3000 mile intervals on mineral ("dino") oil, as amost any "normal" driving is in the catagory of "Severe" these days -- highway driving is mostly 60 mph + (80 + on interstates these days), there is always way too much dust in the air, etc.

Synthetics can go 6000 or until the soot is too high for deisels -- which means the synthetic is the same price, by and large, with only half as much used, saving petroleum.

Mobil Delvac 1 is sold these days as "truck and SUV oil", 5W40 and is suitable for any engine. Probably the best bet in most vehicles, gas or diesel. Mobil 1 5W30 will be better in gassers in cold weather.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!