Could be wrong but what I'm thinkin is what you've mistaken for engine oil could actually be unburnt fuel residue - indicating spent or stuck-open injector tips pissing fuel into pre-comb chambers that can also have fouled the chambers.

If you swap the injectors and then #1 and #5 remain clean but oily injectors carry the problem with them, then I'd be testing those injector tips.

One caution - pulling and swapping injectors around might require new seals. This will be obvious because fuel will be leaking where injectors fit into the cyl head. Paint mark the suspect injectors too - so you dont lose them swapping em around.