f12 what type of gas can I put in there?
First you should make sure that the lubrificant oils in the compressors can be used with the new gas and second they should have similar working pressures and temperatures.
To retrofit with R134a you have to purge the compressor and system of the old mineral oil as they're not compatible and deep clean everything with an appropriate solvent. Then recharge the POE oil (compatible with R134a) and around 15% less R134a of the original charge of R12. This will work nicely but at a reduced efficency (longer cooling times and increased electric expense!).
A much better replacement is R409a (but still contains HCFC), purge the system of the old R12, recharge with around 15/20% less of R409, check and in case add a little more and you're done! (Mind that pipes will not have to freeze near the compressor!)
It might even work better than with the original refrigerant as it has a much higher heat transfer capacity!
A newer alternative is R-437A, also called Isceon 49plus from Dupont, drop in replacement too, same advice as R409. No need to change oil.
