there are other possibilities also. Have you changed fuel suppliers,
or maybe gone from avgas to mogas or mogas to avgas? Some fuels burn
a bit more slowly than others as a function of just what the actual
octane is. If burning slower, it will create more carbon in the
exhaust, go past the ring gap,then thru the valve guide clearances.
Or, your mixture control could be screwed up, and it's not leaning the
way it did.

In general, it's good to know what the usual behavior of one's engine
is. Black oil soon after change in some engines isn't always evidence
of a problem, I drive an old radial engined beast, and even after
overhaul, with the clearances and low normal compression, the oil is
black in 2 hours, tops. the thing having been designed to run on 70
octane doesn't help, either, with modern hi-octane only. Just thought
I'd mention this in case someone else who has the problem panics
before they find out it's normal for their engine.

Looking in the rocker boxes is a good start, however.