Actually R-22 circulates mineral oil quite well; that's the combination used in the central a/c of your home. You don't want to use R-22 in an operating mobile a/c because it's the wrong refrigerant. The pressures are much higher. If you're lucky the high pressure switch will just shut the system down. But you may blow out a hose, or the condensor, or damage the compressor.

The idea behind using R-22 with nitrogen is that a combination of 5%R-22/95%N2 is not a "refrigerant", and therefore can be vented without violating EPA rules. That's probably bending the rules a bit. As well, in the murky haze that is my memory I recall that R-22 is very easy to detect with an electronic leak finder. Hence perhaps 5% is enough.

Personally I like the dye method. Charge up the system, ignore it 'til it don't work again, go look under the hood. My '93 124 had a slow a/c leak - about a pound every year and a half. I used the above technique. It was easy to spot the leak. The system was spotless, except for the expansion valve - it pretty much glowed in the dark. Replaced TXV and all is now well with the world.

My $.02,

- JimY