The seal fits into grooves cast in covers, these grooves need to be pretty clean, any crud and the oil may find it's way round the gasket.
Clean groove shouldn't need sealant but used sparingly it should be ok.
I can't see any other way for oil to get into spark plugs other than poor gasket sealing, or combined with a duff PCV.
FWIW I used red non-setting red sealant in my grooves (and rubber washers) after gasket and PCV renewal and it was oil free months later, and I don't recall focusing on the grooves much.
With new PCV and a smidgen of sealant it should cure the problem.