I'm not an AC guru, but have been told that if you mix the 2 (R12 & R134) the different oils will gel and stop up the system.
You should still be able to get the R12 installed by a lic. AC guy, or you can update the whole system to the R134 , They sell a kit for about $30.00 that contains all the parts needed and the oil and R134 refrigerant.
I did this to my 92, and it has been fine (other than the slow leak in one of the hoses
).
I did remove the compressor and drain out the old oil before getting a vacumne pulled on the stystem, then just added the supplied contents, and it's cooling fine.
Hope this helps,
__________________
Huck
Subaru Ambassador
92 SVX LS-Tour Magnaflow Exhaust, 5-Spd-AWD
88 XT6 AWD 5-Speed "Bride of FrankenWedge"
15 Impreza Premium Sedan
15 Crosstrek XV 5-Speed
My 5-Speed "How-To" Write-up
1976 Pontiac Firebird Formula
Current Count of Subaru's Owned.... "70"