I am by no means an expert, but here is what I learned.
R12 uses mineral oil, R134 uses ester oil.
I don't think there is a place to check the oil level on a 300E. The oil mixes with the refrigerant. No refrigerant, no lubrication equals compressor seizure. You may well have a fried compressor.
If I were you, I would pay for a diagnostic test at a shop, where they charge the system, add dye and see if it works.
Probable culprits: compressor to manifold pipe joint, expansion valve, or evaporator. The first two can probably be detected with dye. The evaporator needs a "sniffer" at the vents.
Good luck - you are heading down the slippery slope. Lot of posts on this subject.
Chuck (AC but No Money)