Best thing to do is find a friend that has some AC gauges and run the system to check pressures if its feeling a little weak. If it has worked good in the past and just started to feel weak theres a good chance you have a slow leak and just needs more freon. I'd be surprised if they system still has R12 in it as most have been converted to R134A (atleast in this area) if the service ports take screw on style fittings its still R12 if they take quick couplers its R134A
Add oil. Not really unless you removed it or it leaked out. No way to measure the amount of oil the system. Proper way is to flush the whole system, recharge and add proper amount and type of oil. It is a fairly big job. Rarely needs done unless there was a problem. R12 vs R12a? not sure on this one. Maybe you are thinking of a R12 replacement product? Many of these are a hydrocarbon like propane or butane. I don't suggest them.
First thing to do is blow out condenser, evaporator and fitlers. Condensor is usually near radiator. The cooler that cools the R12. The evaporator is what the air blows through cooling the air. It is after the blower fan. Blow them all out very good. Evaporator can be hard to get to but often is plugged up with dust over years of operation. Sometimes you can kinda get to the evaporator through ducts. Look through them with a flashlight and you may see the radiator looking thing. If a lot of dust blows out when blowing through ducts I would get more serious and get to the evaporator and blow it out very good. If the condenser and evaporator can't transfer heat, the system doesn't work very well. Next I would suggest hooking up gauges and running in warm to hot weather. Making sure pressures are good. It may have very small leak and need charged up.