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50-1 is stout for that old engine and you may be leaving oil when burning the fuel that is building up in the top end. Do a head check for bolts and a compression check as you may have a blown head gasket. Clean it up and run 128-1 ratio and see if you don't clear it first. You don't say if the oily area is near the exhaust manifold but if it is, once again could be un-burnt oil from high ratio and of course a loose manifold gasket.