You should reclaim any 134a when you intend to dismantle the system. Putting foreign chemicals in there will mean that you can only vent it, which is illegal. It is unlikely that you would be caught, but remember that it is a $25,000 fine and possible jail time.

As for the oil, the only thing that you can do short of evacuating the system and flushing everything is to assume that your oil is good and the quantity is correct. There is absolutely no way to measure it in-situ.