I wonder how oil could get into the pre-chambers. Even if excessive oil in the cylinders, seems it would be hard to get into the pre-chambers since average flow is the other way. Sure it wasn't diesel fuel mixed with carbon? I don't think diesels have much problem with oil getting past the valve guides since they don't have a high intake manifold vacuum to suck oil in like gas engines do when idling. I've pulled the head off my failed 1985 engine ~10 yrs ago, also removing the camshaft assembly. I don't recall rubber valve stem seals, but then didn't look for them.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans