It is the oil used with R12 which will not work with R134 and the other way around.

Also "O" rings used for R12 (A/C line fittings) will not work with R134. And the evaporator "expansion valve" may need to be replaced as well as "flushing" all the old R12 oil out of the system..

This subject has tons of info on the internet. Go to google.com and search for the words...

convert r12 to r134