When I installed my A/C from a wrecked car, it would bog down bad. I thought I just overcharged it with the 134 conversion kit. I let off some pressure and it fixed the bogging problem, but never really cooled too well. Now two or three years later, I hooked up the gauge that came with the conversion kit and added some freon, works great and doesn't bog.

It would be easy to change the oil, but you would have to have it evacuated at a shop first. Then either have them refill it, or DIY.