Correct way to recharge AC 72 Duster 340 with Sanden 508 compressor

If you had a friend who does commercial AC you could pressurize it with nitrogen to 150 psi and let it sit for a day, should be no pressure drop unless ambient temp takes a swing.

Ester oil w dye is good. It's compatible with any minor trace of mineral oil AND 134a. I'd pull the compressor and dump the oil, add 6 oz new Ester and maybe a 1/2 oz. Like Darter said, black oil is not good, sounds like it got oxidized or the system was open for a long time. Oil pools in low spots when velocity is low or the oil is cold. When the load is high w high condensing temps, the oil will make it's way back to the compressor.