Replacing A/C seals

I've got a 1991 camry that i just purchased. The A/C system had a leak in it and the previous owner did not want to have it repaired as he didn't want to spend the money. I'd like to get the A/C system up and going again this summer. I was going to start out by replacing the high and low side schrader valves on the service ports and all the O-rings. I can replace the valves no problem, but for the O-rings can i just go to the parts store and by some generic O-rings and use those or do they have to be a special kind? Also, i'll probably need to add some oil to the system; what kind should i use with R-12 refrigerant?