You're not going to suck the pan dry just going to a 10mm pump. You could run the oil level a little higher if you want to. Lots of sand guys run shortened pans and still run the same amount of oil, it's just higher in the pan. Where did you get the specs. on the pumps? thats pretty neat. I don't remember the 10mm having 85 psi at any rpm. As far as using a used oil pump, on some motors all you can get are rebuild kits, such as international. Also by the time you buy rebuild parts such as the seals and rotors or gears you can sometimes buy a new pump. On the subie it's quite a hassle to redo a oil pump just because you were trying to save a buck and unless you use a oil pressure guage you may not know what your rebuilt pump is doing. As someone else already stated, it's easier on the oil pump and the water pump to do it while the motor is out. I ordered a new pump on ebay and got a 7mm pump. I ordered from someone else and got a 10mm pump. Needless to say the 7mm guy got sorry feedback.