R12 systems that have been converted to 134a MUST use ONLY ester oil. PAG oil, commonly found in oil charge cans intended for use in systems that were 134a from the beginning will cause major problems if used in converted R12 systems. It will attack the O rings, seals, and possibly even the hoses, react with the remaining mineral oil in the system, and eventually cause death by black goo. Read the label VERY carefully, and, IF IN DOUBT, LEAVE IT OUT. Oil charge 134a with ester oil does exist, but unless you have a major oily mess somewhere at a fitting or the compressor seal etc., you probably don't need the extra oil. You can try adding straight 134a, but be careful not to put in too much, as there is a possibility of blowing something with excessive pressure. I would recommend that you try a can of plain 134a, some of which come with a hose and gauge attached.

You probably have no idea how the 134a conversion was done on your car. The correct way is to replace all the receiver/drier, all the O rings, the orifice tube and possibly even the hoses, install conversion fittings and a label, and flush all the old R12 mineral oil out of the system and replace it with ester oil. However, a lot of conversions have been done quickly and cheaply by evacuating the system and recharging with 134a that contains ester oil. This will work OK for a while, but is prone to leakage, as the O rings tend to deteriorate when exposed to the new type of oil.