Replace the oil filler cap, the gasket has gone and oil will leak out when thrown up there by the cam at high rpm.

Will only leak badly on the highway, can increase oil consumption to a quart in 1000 miles.

The only real clue you will see is that there is aways some oil in the drain area around the filler cap.

I know, I've replaced all of mine now!

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!