The Mineral oil will be there until you completely flush the system. There will be several ounces hiding in the evaporator, condensor, drier and any place that has a low spot for it to pool.

There are other oils out there now if you don't like ester but I would not try the PAG if it were me. The cleanup of a gummed up system is a nightmare on some cars.

I bought a 1/2 gallon of ester at NAPA and never looked back. I use it as needed in any car.