Oil pressure Perkins 4108
I have a 1978ish Perkins 4108 with an interesting problem. The engine will run to 15 to 20 minutes when the boat is actually moving and then the low oil pressure alarm goes off. I have run the engine at the slip under load up to operating temperatures and then 15 minutes no problems with the oil pressure. Once the low oil pressure alarm comes on there is initially no signs of over heating and then suddenly the engine will overheat. I have drained the oil, which was very dirty/sludgy, and I have not yet replaced pending checking the oil filter. I have no records from the previous owner regarding oil changes, filter changes etc. Suggestions as to the problem?