John,
     You may be correct about all this,  but I think the kit at Advanced
Auto,
or Wal-mart, has something special in it that makes R12, and R134a oils
compatable.  I've converted a Jetta, AC system.  I rebuild the compressor,
and replaced the condensor at the time. Vacuumed out the system, and that
thing will freeze you out, today.  That was 3 years ago.  I'll be doing
another one here soon.
                                                      Eric

John Connolly wrote:

>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.
>
>
>
>