This is a huge problem for everyone... you are not alone....all the pros are in the same boat... if you have a leak .. it is assumed that oil was taken with it... if you have the Delco compressor it does not even have a sump like the old York... so all the lubrication which it gets is from it being dissolved in the Refrigerant and moved around the system...
The only way to know for sure on a system new to you is to flush ( and replace the usual things ) and measure the amount of oil into it.... same with the refrigerant.
Since the oil amount is very important.. not only on the low side.. but too much is very bad also... and will overwork your compressor and adversely affect you cooling in short order.. thus you can not just add oil to be on the ' safe ' side...
It is a bummer of a situation all around... but done right a system has the potential of lasting a long time with no further problems...