Can anybody please advise if the valve on the compressor (on 1986 300E with M103 6 cyl. engine), is low or high pressure? Also I have bought a R12 to R134a conversion/recharge kit and the instructions state that you should just add the new ester oil and that the existing oil does not have to be removed because it would just sit at the bottom of the acumulator. My system has a leak and at this time has very little R12 can I just evacuate by pressing the high pressure valve and letting the remainder small amount of R12 gas hiss out??? Has anybody tried this?
By the way R12 is illegal here in Canada – any shop using it is subject to loosing their license, fine and up to 15 years imprisonment. I know it’s silly to do this in Canada while our southern neighbors are still polluting but this is where I live.

I have cut and pasted part of the FAQ instructions --
9. Do I need to remove the oil that's in the system? Is the old oil compatible with the new oil?
No. The residual mineral oil left behind after you evacuate a R-12 system will not mix with the new R-134a refrigerant. That is why we add Ester Oil, because it will mix with R-134a, lubricate the system components and is compatible with the residual Mineral oil.

The mineral oil just collects in a low place in the system (such as the accumulator), where it stays, until it is removed at some later date during future maintenance or repair. The mineral oil does no good, but it does no harm either. It's just there.