If its not too late, go in with synthetic compressor oil. It'll extend the life of the already crappy compressor and if you have to switch refrigerants, all you'll have to do is vacuum it down, as synthetic oils are compatable with both r134a and r12 (freeze 12 too I'd imagine). Plus since you've already done all the hard work, its not that much more to get your license and get r12. That's what I did and I'm very pleased with my system. Next spring I'm getting my windows tinted then I'll be golden.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo