Add some oil. If you don't, the pump could lock up.
Mineral oil is used with all R12 systems.
Why would you want to even consider doing this? R12 is no longer in production and as such it cost almost 4 times as much as R134. But to answer your question, use R12 in a R134 system and you will destroy the compressor as the oil is not compatible with R134 oil.
It's only Freon if it was manufactured by DuPont, but to the important things... Your 89 would have had an R12 system from the factory. You can't use R134a in an R12 system unless you do at least a partial retrofit (evacuate all the R12 and compressor oil, replace the orifice tube, replace the accumulator, replace the hose connections, add PAG oil and R134a).
Don't. R12 is so very expensive that you should have your vehicle converted over to use R134. You have a leak or you would be needing to add refrigerant. Have the leak repaired by a professional and have the system converted over to R134. The will remove all R12 & the oil from the system and replace it with R134 & the correct oil. They should pay you for the R12 due to how valuable it is. You may end up spending very little on this repair.
There are different types of refrigerant oils. Refrigerants such as R22 and R12 usually use mineral oil. But some of blended refrigerants like R410A can sometimes use a poly oil. These oils can not be blended together. So a system that already has a refrigerant with mineral oil must be charged with refrigerants with the same oil.
You do not want to buy Freon as it is very, very, expensive. Have the leak repaired, the system purged of all Freon R12 and the oil. Have it converted over to R134 refrigerant. You cannot just add R134 as the oil used in R12 is not compatible with R134. You will be money ahead by converting it.
The simple way is to remove the R12 and install R134A. by a kit with that will have a hose, R134A with oil. The new oil may plug your leaks. or go to Mexico and cross the border and buy R12 in 12 oz cans The border guards may take your R12 cans when you come into USA. or Drive your car into Mexico and have them fill your system with R12.
Freon R12 refrigerant is so very expensive that I recommend you have the A/C converted to R134a. But first you must have the leak repaired. The shop will then remove any R12 left in the system, flush the system to remove the old oil, install fresh oil that is compatible with R134a, and charge the system. If there is any R12 left they may pay you to recover it.
easy being an open drive unit use ester rl 68 oil for r134a refrigerant or es32 mineral oil for r12 refrigerant.
Drain the r12 and put the 134 in. You can flush the system if you want to, but it really isn't needed. You will want to add some oil for the 134 in addition to the gas. You don't need to worry about changing O rings or anything else.
First of all, you don't use R12. Rather, R134 which has the oil included.