Spark Plugs - Oil Fouled Casuing Misfire?

[Teocali]

Hi all,

Looking for some guidance. Background is misfire (ECU shows cylinder 3) - cyclinder 4 spark plug seems to show oil fouling?

Astra - Oil Fouled? by mongooseteocali, on Flickr

Where the other three show ash deposits:
Astra - Ash Deposits by mongooseteocali, on Flickr

Some light mayo on the filler cap - planning to check compression tomorrow.

Game over or change spark plugs and re run? What could be causing the deposits / oil?

Appreciate any comments, Sam

[Teocali]

Thank you both.

Would I be correct to assume that a compression test will tell me whether it's the pcv or seal stems/bore wear?