Unless it's been converted to R-134a, it's an R-12 system.
There are a few refrigerants that can be added alongside R-12. Freeze12 is one of them.
Beyond that, you convert it. HOWEVER, instead of going to R-134a, which BTW, worked very well in my MR2, an even better refrigerant is ES-12a, a hydrocarbon refrigerant. You can find it on es-refrigerants.com.