[red82gt]

This is the info I got from a guy named Glenn, who posts a lot on the corral and works in the A/C field for a living.
He recommends flushing the condenser and the compresser and replacing the liquid line. Replace all the o-rings (It's not difficult, maybe will take an hour or two), have the system vacuumed out and then charge it up using the combination oil that is designed for the retrofit kit. R12 uses a mineral oil and r134 uses an ester oil. These two are not compatible with each other and this is why you should flush the system and use the retrofit oil. The r134 will be able to escape right past the r12 o-rings and oil so unless you want to add a can of r134 a month, change the o-rings and use the right oil.

Also, if your A/C hasn't been working for a while, you'll want to replace the dryer or else the moisture will cause compressor failure within a couple of weeks.

As jimberg stated, there are drop in R12 replacements that work better than R12 and much better than R134. If I didn't live in Canada, I'd probably be using one of these but since they're hard to come by up here I'm stuck with R134.