Sorry to be late to the party, But:
Pull the return to tank line and run it into a clean bucket. Run the engine for a few minutes.
If you have oil traces, then it is getting into the head passages through:
a casting porosity, a loosened internal plug, a leaking small gasket. (The engine oil
pressure being greater than the fuel return to tank pressure.)
Either way, the head needs disassembled, pressure checked under water and the leak found.
Or R&R exchange for a rebuilt head.
Miz