Brother of The Benz, can do,
First, there are no seperate oils, the compressor oil is in suspension in the freon and circulates with the freon.
To check the oil level you close both service valves(if you have them) and remove the oil fill plug at the rear of the compressor. Using a probe, gauge the oil level. Remember if you have been running with a low freon charge then the majority of the oil will be trapped in the evaporator resulting from the freon not boiling over to the return line and back to the compressor.
This is why we always charge the system before we add oil for the oil of the fully charge unit will be circulating with the freon.
Refrigiration oil can be purchased from any reffer supplier.
The weights I referenced are unique to A/C oils.
If you are charging an empty unit freon and oil combined in the same container is available.
Good Luck it really is simple.
Happy Trails Beep Beep from The Spiderman in Houston!!!