I had that happen from a blown exhaust gasket. One of the gaskets inside the fan cowling.
503's seem to have a lot of oil in the exhaust and the fan blows it through the fins on the heads.
I had taken the exhaust Y off to check the rings on the exhaust side and only replaced the 2 outside gaskets between the fan cowl and the exhaust Y.
I thought I would get away with not replacing the 2 inside gaskets between the cylinders and the fan cowling being that I didn't take the cowling off but no.