Introducing PAG oil

[Trio3b]

99 Buick Lesabre. Looked for a mobile AC forum but maybe missed it.
So AC working fine until recently. Gauges showed no pressure hi or lo about 2 mos. ago so system has been exposed to air and probably moisture although we're very dry here in southwest .
Yes there's a leak that needs to be found first.
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*** IMPORTANT***

I will probably sell this car so I appreciate suggestions to buy a new compressor/rec-drier, condenser kit but thats not going to happen. I'll be lucky to get seven hundred bucks for the car here near the border, so I'm not worried about whether I destroy the AC system. Obviously I would rather not. So . . .

I will charge system with compressed air to activate the pressure switch and confirm the climate control , compressor and clutch are working by turning AC on FOR A FEW SECONDS ONLY !! There should be enough residual oil to protect the compressor for a few seconds.
Once I confirm all that is working I will locate leak, repair and then evacuate system and charge with the proper amount of 134a.

Here's my question. I know when systems are opened up and replacing parts , it is recommended to spread PAG oil throughout the system pouring a little here and there up to recommended amount then pull a vacuum. Also it is recommended to hand turn the compressor to spread oil.

In this case I will NOT open the system unless I find a huge leak and even then I will only fix that part and bolt it all back together. So assuming something minor such as a loose fitting or a slow leak, once I get that fixed, I will pull a vacuum.
1. Does pulling vacuum remove any and all previous remaining PAG oil? I'm sure a portion of the original oil was carried out through the leak that evacuated the system in the first place.
2. My guess is there will remain a little oil in the system. If so I'm assuming I'll be adding some PAG oil. I have seen PAG oil injectors. Are these necessary? I would like to add the oil without introducing air to the system.
I have seen people fill up the yellow intake line with PAG oil by letting it hang down in a loop and then pouring it into the line until it exits out both sides. This way there's no air in the line prior to the first charge.
3. Will the oil reach the compressor through the manifold gauge set and low pressure side of the system in time to protect the compressor when it kicks in adding the first can of freon?. If not, then where is the best place to open the system and add a little oil to protect the compressor?..

I'm really using this vehicle to practice on so that I can do a proper recharge with a new AC compressor kit from RockAuto on my other Buick which I will be keeping.
Thanks for any advice

[Blue Bowtie]

Before running a compressor at speed, grab the clutch hub (not the belt sheave) and manually turn the compressor several revolutions to check for binding, smooth bearings, unusual noises, etc.

Instead of adding compressed air to the system, the low pressure cutout can be jumpered temporarily for testing.

Applying a vacuum to the system to boil out air and moisture will not remove the system oil charge unless the vacuum is applied at a low point and oil is collected there. It will (slowly) boil water out of the oil. However, PAG refrigerant oil and R-134A refrigerants are soluble, and is likely to be lost with the charge due to a leak. Some oil might remain in the compressor case for a while withhout being carried away by refrigerant.

The charging (yellow) hose can be filled with oil to dispel air and inject the oil into the system. The total volume of oil added is very slight, however. If topping off a charge, it may be worth considering the addition of PAO oil instead of PAG. They can be mixed, and the PAO is more stable and less subject to loss.

If the system is completly empty/new, oil can be added to the compressor before installation and added to the accumulator to make up the remaining oil charge volume.

[Trio3b]

I would add compressed air to aid me in locating a leak in addition to triggering the low pressure switch. Yes I have heard about jumping the low pressure switch however it's my understanding that some systems can be be damaged by doing this. I'm assuming , the first can of freon will bring the low side up to the minimum 25 to 30 pounds of pressure to trigger the compressor clutch.

There seems to be tons of info on the net about pressure and humidity charts, and operation of manifold gauge sets but very little on introducing PAG oil properly and or flushing systems. Do you happen to know what inexpensive product can be used to flush out mobile A/C system's? Everyone seems to be very hush-hush about the chemical composition of the flush itself. Also is there a definitive information about what year condenser passageways became too small to effectively flush and should be replaced with a new condenser?

Thanks for the info