PAG oil is 100% synthetic in manufacture. Ester oil is a modified blend of organic oils. The viscosity of the oils is rated in centistrokes at 70 degrees (F). There is not much of a way to tell what the high side or red gauge will read. This depends on ambient temperare, refrigerant charge in the system, cooling fan efficiency, condensor and evaporator cleanliness, airflow through the coils, etc. R-134 will work with either Ester oil, or PAG oil. Should you use PAG oil, the sealing "O" rings in the system must be changed as the oil will eat them on short order. If you use ester oil, I'd still replace them as they are aged and probably leak. The accumulator/dehydrator, or receiver/drier must also be changed if the system is converted, or has been left idle with no charge in it. They are not to be reused. Being an older unit I'm making the assumption the compressor is a York style two cyliinder piston compressor. If so they are the best, (my opinion). You must however remove the compressor from the engine, (no matter what type) pull out the drain/fill plug and drain the oil out of it into a calibrated beaker for measurement. Compare this to specification(s) for capacity. Refill the compressor with PAG, or ester oil in the amount removed. Do not add more unless the system suffered catastrophic failure such as a ruptured hose etc. When the compressor is refilled, set on the bench and turn the clutch and compressor 10 full revolutions clockwise to distribute the oil onto the parts. Tape or cover the openings to not allow dirt to enter the crankcase. Remove and throw away the accumulator or receiver. These parts are not to be reused. If you don't know the difference between the two, a receiver/dryer is in the high side on the outlet path of the condensor coil, and accumulator is on the outlet side of the evaporator inside the dashboard. Most receiver/dryers have a sight glass in the top also, Accumulators do not. Remove, and/or check the expansion device for cleanliness. If an orifice tube, in the trash it goes. A thermostatic expansion valve unless suspect can be reused. Before reconnecting anything ascertain how much oil the system requires. This can roughly be figured by following this formula: Oil amount (oz.) = [(Refrigerant charge in oz. x 0.06) + 2.2] ÷ 0.9. I've used this for years myself. You have to have enough oil in the system to circulate with the refrigerant but too much and you will slug the valves out of it. Too much oil also acts as an insulator reducing cooling effectiveness. Citing that you have already filled the compressor, if the system uses a receiver/dryer, dump one ounce into this new part, and the balance into the evaporator core inlet. If an accumulator system, do not dump oil into the evaporator but dump the oil into the accumulator. Reconnect everything using all new polyurethane "O" rings, and seals. Once everything is reconnected, ensure the condensor core is clean with both an air gun, and garden hose. This is above important and one of the most widely skipped steps in maintaining an automobile. The cleaner the better here. Pull the a/c system into a 29.75 inches of vacume for at least 45 minutes. Shut the vacume pump off, close your manifold valves and ensure the system hold this vacume for at least five minutes. If it holds, administer refrigerant into the system high side as a liquid, but don't exceed one pound. Stop the charging, allow the liquid refrigerant to "boil off" into a gas. you will see this as the low side pressure gauge ascends to about 50 psi. Wait a couple of minutes, Start the engine and run at 1500 rpm with the doors open, blower on high speed and start to administer refrigerant into the low side of the system as a vapor. The compressor may cycle off and on until the system charge elevates enough to satisfy the low pressure cutout switch, or you can bypass this switch if you know it works. Continue to charge until your low pressure gauge is indicating about 35 degrees denoted on the inner temperaure scale toward the center of the gauge. High side pressure can be as high as 350psi but if that high the condensor coil is partially plugged. This is very generic in nature but will get you in the ballpark. Rob