80C and just a couple of degrees higher, seems to be the average temperature of most 603 cars discussed on this forum. Consensus is, that heat is the enemy of the cylinder head. Closer to 80C is better.

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1987 300D 146K Original #14 Head
One eye on the road, the other on the temp. gauge

Gone But Not Forgotten
1983 380 SEL Long gone
1983 300SD Wish I hadn't traded it in on a 90' Corvette
1989 300SE My all time favorite
1995 E420 Went like a bullet