The refrigeration oil used in an R-12 system is NOT compatible with that used in a R-134A system, if changing refrigerants, change the oil as well.
Good methods nc pearson, but with regards to the oil, the only problem with the mineral oil in the old R12 systems is that it is not miscible with 134a; it cannot be dissolved in the refrigerant. bigger systems rely on the oil being carried through the system with the refrigerant, but on little systems like these little or no oil moves out of the compressor, so miscibility is not required. That is why you can drop 134a into a system without having to switch to Polyol Ester oil. Also, while weighing the refrigerant in on a scale is technically correct, in practice it often isn't practical. A system can also be charged using suction pressure and temperature measurements. I rarely ever weigh a charge into a system that small, and even if I do, I usually end up adjusting the charge to get it running the way I like it.
Because the temperature and pressure characteristics of R12 and R134a are so similar, the two refrigerants are compatible with each other too. Although it is certainly not "best practice" blending the two will do no harm. In a pinch you could
top up an R12 system with R134a if there was no means of evacuating available.