how does used motor oil get into your transmission? This is how it sounds to me when I read it.
If you are leaking motor oil from where the trans and the engine are mated - there is a GOOD change it's the rear main seal, however I would look into all other options first to ensure things are hooked up and working as they are intended so that there isn't excessive pressure forcing the leak.
That aside - if it's gear oil, it can be dealt with very easily. 1 Gallon of gear oil and top off as needed. This is how my 87' 9000 was - went though 1 gallon every 3 months. The fluid was a clean as it ever could be - and I knew it needed fluid when first and second gear needed a little effort to engage

Aside from that you can run a 9000 gearbox on pretty much no gear oil and get around - previous owner of my 87' must have done this for months if not years.... gears all worked perfect and never popped out either.