I think this series inline six had some problems with the valve guide seals or valve stem seals (?). Losing 500 ml every 1500 miles is about a quart every 3000 miles. Not bad, yet. My '77 300D was at a quart every 600 miles, but it could have been from a lot of oil leaks. If the car sat for a week or so without being run, the oil level still dropped.
Check your oil before and after long trips. That should indicate if you're losing oil though blowby. On my '77 300D I saw puffs of smoke if I removed the oil refill cap and then started the engine. Mine produced a cloud of smoke a couple feet high. Does this indicate blow-by?
Anyway, I later sold the car. It's still running, as far as I know, though the owner probably carries a jug of oil in the trunk
Robert W. Roe
1984 300SD 165,971 miles
[This message has been edited by Robert W. Roe (edited 10-13-1999).]