Something else to look for. Most of these era Merc engines do not have a PCV valve. They have a fixed metering orifice, which can evetually clog. During operation with manifold vacuum, ventilation flow is controlled by the orifice. The system draws clean air from the air cleaner housing - into the engine via the cam cover nipple and back out through the metering orifice to the inlet manifold. During WOT operation the flow can reverse and blowby is vented to the air cleaner housing.

If the separator is not functioning properly oil can be blown into the air clearner housing and then ingested. Also, if the orifice is blocked flow will be "backwards" all the time instead of just at WOT.

Duke