Vehicle make is 1987 Toyota Supra 7M-GE Manual transmission 55K miles. My car has been sitting in a climate controlled garage for 22 years and has periodically been started and driven limited to two times each year. I have not added R12 Freon in this time period and the system has been empty of R12 Freon during this time. I purchased a new Receiver Dryer to install before adding new R12 Freon. My question is; must completely new mineral oil be reinstalled into the AC System or will the current mineral oil already in the system be fine to continue in use? The ac system holds a negative vacuum and is currently absent of leaking. I believe the R12 slowly leaked out from non use over an extended period of time. I know I must reinstall approximately 8 tenths of an ounce mineral oil with the new Receiver Dryer installation. Also, please give any additional opinions on the recharging and servicing of the ac system that would be beneficial to correctly performing the service. Thank you for your time, Mark
Long-term it would probably be best to convert the AC system to R134a, but if you have a supply of R12 available and want to use it I would probably flush all the old oil out due to the age of the system and start clean. Also if the AC system has been exposed to atmosphere for a long time the oil will probably have absorbed some moisture and possibly other contaminants, though it is not nearly as hygroscopic as the PAG oil used in R134a systems or the Ester oil used in conversions.
Note that the system may leak under pressure even if it holds vacuum. I've seen it happen. (I usually use an electronic detector to check for leaks but UV dye will also do the job.) Be sure to replace and lube any o-rings. Might be a good time to replace the hoses if original. A good hose shop can make up new hoses with your fittings if replacements are not readily available.
