Look at your spark plugs.

If oil fouled, can be only two causes -

Worn valve seals

Worn piston rings, or both.
When taking out the plugs, note the position of
the oil or carbon fouling, if any.

If they are more towards the lower portion  of the insulator, which would be facing the piston top,
it's very likely the piston rings.

You can judge it from the direction the side electrode
is pointing.
If valve seals, at an early stage of looseless, there
will be some bluish smoke on starting and initial
run, which goes away after driving a bit of distance,
as the oil is burnt up.

Loose piston rings tend to emit whitish exhaust
perpetually, particularly if quite worn.