With AC compressor's lubrication there's no such thing as educated guessing. Even more if your compressor is of the vane or scroll type. Lack of lubrication, or, on the other extreme, hydrolocking, kills these things.
You have to recover the refrigerant charge, remove the compressor and flush the system (you haven't experienced a compressor seizure, so you will flush just to remove any leftover oil and this will allow you to flush the condenser too, and the closed loop refrigerant flush allowed by an RRR station with the proper flush kit could be enough, but it's still good policy to replace the receiver dryer each time the system is open).
Then you'll have to follow any compressor installation instructions you have, but it's generally take the compressor, drain any oil from it and measure and then insert the system's oil charge inside (if you have a drain plug, remove the drain plug and drain and then refill oil of the proper ISO VG from there, otherwise pour it in the suction port and rotate the clutch hub so that it gets drawn inside; double end capped PAG is the best option since you won't have to worry about moisture) then install the compressor on the car and rotate the compressor's shaft (rotate pulley if clutchless, rotate clutch hub if with clutch) by hand for a lot of times (20 full turns to be fully sure, though 10 might be enough), so that any excess oil gets discharged in the high pressure line and won't hydrolock the compressor upon startup. Then thoroughly vacuum, and finally charge the system up with the required weight of refrigerant (recommended: liquid in the high side, by weight, with compressor obviously off). Again, turn that compressor shaft by hand for 10-20 more times to be fully sure the compressor won't hydrolock (better be safe than sorry). After you're done let the system rest for 10 minutes and then start the engine, be sure it's in idle and then engage the AC compressor and let it run for 5 minutes keeping the engine in idle. Compressor installed, properly lubricated and ran-in, with a full refrigerant charge. Now enjoy your ice cold AC.
By all means, don't let the engine turn the compressor without any oil inside it.
That's the proper way of doing things and not risking compressor seizure due to "too much" or "too little" of anything.
The oil injector thing is to be used only to replenish whatever came out during refrigerant evacuation. That's also what the canned oil charges available in the USA are meant for.
Hint: If you haven't got an RRR (Refrigerant recovery & recycling) station, let someone else who has one (and is licensed) do the job, otherwise you can mess things up pretty easily. By all means forget about refrigerant cans and canned oil charges.