Published info states R-134A is not compatible with the mineral based lubrication oil in a R-12 system. R-134A requires PAG oil.

I tried the R-134a conversion in an R-12 sytstem and was not satisfied. You will lose cooling capacity in an older R-12 system. The condenser is under-sized for the higher discharge pressures. Expect loss of cooling capacity the hotter it gets outside.

My recommendation would be to find some old stock, R-12. If none available there is a new blend called R-401A or "hot shot" that i hear works well and it is compatible with all types of lubricating oil.

It is always a good idea to thoroughly flush entire system, remove and clean the TXV or metering device, and leak check thoroughly. You will not want any leaks if you buy some R-12. It is expensive!