Oil doesn't flow upwards, what oil can accumulate in a cylinder from a
bad piston ring?
If any lube oil is accumulating, which is doubtful, it's seeping down a
valve stem because the plastic valve guide seal is worn. But doubtful.
Suggest you add some Techron or similar injector cleaner to the fuel and
check that all the glow plugs are working.
Cold starts are the true test of the injectors, any minute bit of carbon
or dirt will cause knocking and roughness. Don't get talked into some