Please don't use 90wt. I've used dexron iii the entire time, this is not a recommendation so much as a personal experience I'm sharing. It's worked for me so far.
As far as capacity... It hold around 8qt total, but that will include the torque converter. Most people put in 4.0qt to start with, then crank, then add fluid. If the tc is not full it will slowly fill while the transmission is in n, from what I understand about these transmissions. So set parking brake, put it in n, and crank with 4 quarts. Add the rest as the torque converter fills, adding too fast will apparently cause a lot of fluid loss out of the transmission vent.
As for checking the level, check it in neutral with the fluid up to temp. That temp May take a while to happen in any given application but I always check mine after a good amount of driving. Checking it in park won't supply you with a correct indication from the factory.