You received some very good replies from some obviously experienced and wise folks.
I never can keep myself from responding to these posts.
First of all, it is correct that PAG oil is used with R134, but ONLY in those systems which have never seen R12. For a converted system, use Ester type oil only.
The combination of R134 and R12 is not good, it can coagulate and cause problems. If you insist on changing to R134, it is recommended that you flush the system thoroughly and pump it down for 24 hours.
There is a very valid reason for such a lengthy pump down. You need to get absolutely as much moisture traces as you can out of the system. Moisture combines with refrigerant which makes an acid which will eat up the system from the inside out. This situation is more prominent with R134.
Also, I'm pretty sure you can still legally purchase R12 as an individual, but only in quantities in excess of 30 pounds. Go figure. This law could have changed, but if it did, it was very recently. This means it will take hundreds of dollars to buy 30 pounds to get enough to charge your air conditioner.
And lastly, I have converted a few systems with the opposite results of William. Maybe it's incompetence on my part, but some of it is the fact that in 108 degree Texas heat, R134 just doesn't seem to cut it.
Best of luck,