What can cause small pool of oil on top of piston (engine cold)?

Have owned vehicle for less than a year so know little about history. Runs very well except for excessive oil consumption (1qt/300miles or so). Assumed this was the sticking oil rings problem common to these cars (and Toyota Corollas) but decided to try to determine which cylinder(s) were the problem. After the car sat for more than 24 hours:

1. Removed spark plug from C4 and found everything looking good except for a bit too much white ash build up on the plug (and the plug past due for a change). Put a borescope into C4 and everything looked good to my inexperienced eye (can attach video of borescope and photo of plug if needed).

2. Removed spark plug wire from C3 and found oil in the plug well. Assume tappet cover seals are leaking a bit as top of cover is clean. Removed spark plug from C3 and found it in good shape except for being wet and past due for a change. Put the borescope into C3 and found oil filling the gap between the piston and cylinder and a pool of oil on the top of the piston - probably covering 1/8 of the piston surface (apparently piston top is not quite level). Again, can attach video of the borescope and a photo of the plug if that would be helpful.

Have done nothing further as I am wondering if I need to do compression tests (though not sure how to do the dry test on C3) or if the pool of oil is a conclusive indicator that the valve guides are leaking. Or is there some other possible source for the oil in the cylinder?

I'll have a look into C2 and C1 tomorrow but thought I'd ask the questions right away.