Oil is probably the hardest thing to deal with on a leaker. The amount of oil in the systems vary but are in the range of 6 - 10oz. The amount in a system at any specific time can only be determined in one way: drain and flush and reoil to spec.
In practice oil is considered to be distributed fairly evenly through the system. Recommendations are for adding an oz. when replacing a drier. Maybe two oz. for evaporators or condensers, etc. If the car is having its first leak one to two oz. might be added determined by the type of leak - early Turbodiesels had a low side hose that was the lowest part of the system. The hose often leaked and when it did it leaked mainly oil as the oil lay in the area the leak existed in.
Which oil is in the system is also a major issue. The green mentioned is the flourescent dye that was added to show in a black light. When its visable in white light the leak is very large.
The amount of oil in the system is not a factor if the system is not in use. Keeping pressure in a non-used system is the most important factor. If pressure is gone moisture will enter. Moisture is the worst enemy of refrigeration componets.
Bosch Master, ASE Master, L1
33 years MB technician