If you're gonna run R134a in an R12 system you need a Parallel Flow condenser,
it will make up for the 134's lack of heat transfer ability with it's 30% better
transfer performance.IT WILL NOT BE A "Drop In" replacement.
Spend some time "Plundering Around" on Arizona Mobil Air's website:
http://www.ackits.com/
I'm running BG's FrigiQuiet for an AC lubricant with a reconversion back to R12.