There's really only so many things it can be. Really does sound ring or piston related. Certainly as they are all at the same compression values, or near enough you have to assume that the head gasket is fine, or has blown on all 4 cylinders and letting oil in, which is quite unlikely.
The fact that when you added oil meant the compression values increased really means that you've knackered something rings wise. Not sure how though.
A leakdown test might be worth while, though will probably only tell you what you already fear! If I was you I'd have the head off and the pistons out and check the liners for ovality and also check the pistons for distortion etc. Get measuring!
If you need a hand let me know, I'll be around at christmas, or you can post me the engine in bits and I'll set it up in the workshop at Uni here! :-P