The usual cause of an oil leak is excess crankcase pressure, caused by a blockage in the ventilation system. That is what the person meant by the flame trap being a possible cause - not that it itself is leaking, but if it is blocked it allows pressure to build up in the engine.
First thing is to clean thoroughly all the pipework between flametrap, oil separator box, inlet manifold, and anything else depending on the particular fuel system you have.
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