Given the propensity for 602s and 603s to have a head gasket leak in that area I'm tempted to say head gasket. It's not the kind of head gasket failure that compromises compression or the cooling system. At least not for a while.

Clean the engine particularly under the #1 exhaust manifold runner and see if that's where it starts to get oily again. Pay close attention to whether it's coming from the junction of the head and block or the exhaust runner itself. It could be a bad exhaust valve seal flooding the exhaust manifold. I'd expect a heck of a smoke screen behind your car if it's a bad valve seal.

The chain tensioner looks clean but I can't see the bottom of it.

Sixto
87 300D