I inspected the airbox on my tintop last week whilst I was checking a few other things and noticed that there was quite a bit of oil in the airbox/filter... (the oil breather for the crankcase feeds into the airbox)

I dont notice the oil level in the engine dropping at a ridiculous rate so it cant be doing it all the time....

But there was enough oil in there to soak the filter element and coagulate in blobs..... it has also seaped out around the airbox/throttle body gasket and filled up the webbing on the casting for the throttle body... so there is qutie a bit of oil sloshing around...

This oil wont be doing the TB any good... or the valves..

But should I be worried? What could cause oil buildup like this?

I noticed recently that the engine seems to "pink" a bit more than before.... could it be related?

I'd clean out the airbox and move the breather pipe into a catch tank (old coke bottle for want of a better thing). Then r/v things every day or so.... That will confirm in all likelyhood that it's oil going up the breather pipe rather than anything else and also show how much oil is coming out the breather.

Crank case pressure build up?

Had that on an old XR3 years ago (chav I know). Was suffering with "black death" - everything was gunged up causing all sorts of oil flow problems. Binned it.