Early 9000s did not have a drain to enable you to change the transmission lubricant. We have been adding a drain to them and changing the fluid in the transmission every 30,000 miles at each major service. When changing the fluid the first time, we add the drain and then refill the gearbox and drive for a bit to flush the tranny and then drain it and refill once again.
We use a 1/8" pipe thread plug. The plug must be "fitted" to the hole you drill. Use a 21/64" drill bit to drill the hole. Then do not use the tap like you would a normal tap. Pipe threads taper. You will need to run the tap in until the threads reach the thickness of the case wall then test with the plug. You want to tap the hole until the plug gets tight when a bit more than half its threads are in the tranny. As you change the fluid each time and retighten the plug, it will gradually go deeper into the case. You don't need a lot of torque for the plug to seal. If you use the 1/8" plug enough that it reaches the end of the plug, you can re-drill the hole to accept a 1/4" pipe thread plug. The following pictures give the location we use.