I had a refrigerant guy here charging my chest freezer. the freezer had R12 in it, (the chest freezer was built in 1985) he had to empty the freezers R12 (or just does cause you never know what else someone has put/mixed in a system, even if the refrigerants are compatible or not), and he vacuumed it down and put 416A refrigerant in it. he said 416A is compatible with R12 & R12 systems, but he don't like to mix refrigerants, compatible or not. because he don't know what the guy B4 or after him has done or mixed. (you have to have a license to buy 416A) but he said 416A has less head pressure than R134A and cools better than R134A especially in 100 degree days, it cools like the R12 refrigerant in the R12 systems. as he has used it in his dads tractors/combines and P/Us. 416A is not expensive like R12, you just need a refrigeration guy not a auto a/c man. he said auto a/c shops use R134 cause the auto industry recommends it and the auto a/c shops don't need to know the different refrigerants or refrigeration beyond R134.
I have two R12 systems left, and I will try 416A B4 retrofiring a R12 system to a R134a system. (R134a does not cool the best in a R12 system, even with the right components replaced) R134A is not a direct replacement for R12. with R134a into a R12 system you need to empty the R12 & change some R12 a/c system components and flush the R12 system & vacuum the system down B4 charging with R134a .