I feel your pain, mine does too. It could be a number of things including valve seals, rings, maybe even a headgasket. The only way that you can be sure is if you get a leakdown test and a compression test.

BTW, my car runs great too, but don't rule out the rings. Your oil rings may be finished, but the compression rings may still be good, that's why your car would be running well.

I'm getting a leakdown test this week and I'll get changed whatever is causing the problem. It'll set me back about $900CDN if it's the rings (worst case scenario), but I tell myself that I've already spent so much to swap the engine in myself, why not get this done and get some peace of mind and not worry what my oil level is all the time.

Good luck w/ your engine.