Just a word of warning; most R-12 and R-134a comes with oil (lubricant) in
the proper proportions pre-mixed in the can. THE OILS ARE NOT COMPATIBLE. If
you put in R-134a without a complete oil change (the compressor contains
5-7oz of oil) and system flush, you made a major boo-boo. Ask me how I know.

the systems are compatible with one another, but it requires a time
consuming system flush (I used a flush kit, and Mineral Spirits, along with
my air compressor). You have to take everything apart, and replacing the
o-rings is a wise move since they are already apart. Replacing the expansion
valve and dryer ( a good idea, and a MUST, respectively) round out the job.
THEN you can put your lubricant and refrigerant of choice into the system,
following the proper charging procedures (vacuuming out the system, then a
charge).

If you mix R12 and R-134a oils, you will more then likely find out why this
is not a good idea. The combination does nasty stuff to the dessicant in the
dryer, and the resulting system contaminants will destroy you compressor in
short order. Like I said, ASK ME HOW I KNOW.

John
Aircooled.Net Inc.