I only clean the internal parts during a rebuild. If you have gear oil in it upon purchase or there is an issue, take it apart and clean it with break parts cleaner, and put it back together with at least new gaskets and I-rings. The t-20 takes 1 quart of ATF 3. Many people put an ounce less in and them put an ounce of limited slip additive. The top plug is the fill plug and the bottom is the drain/fluid check plug. The top is vented and the bottom is not. At level, the fluid should be reachable through the bottom plug with your finger. To drain it, the front of the machine must be elevated (winch, ramps, ect...). You can also suck the fluid out with a shop vac attached to a hose or take the t-20 out and turn it so the drain hole is down. Once a year it's a good idea to change the fluids.
l like to buy stuff and no I don't do payments!