It took me a long time to change the transmission oil in my to35.
I plowed a few garden spots so I had the transmission about as hot as it would ever get in normal service. I opened the drains and let it drain over night. By morning no more oil was running out. I have drained my oil in the last few years and the tractor almost never sets out or gets rained on, so any water in my sump would be condensation.
Then I slowly filled the sump with gl-1 mineral oil 90 weight that I purchased from Napa, they had the oil in 1 and 5 gal containers.
It takes a while to refill the sump as the oil has to drain back into all areas of the transmission differential.
System works good, however in the winter the gl-1 oil is thick and the lift is a little slow. But I am in no hurry so I just wait on it,
Probably would have been better to fill sump with 10w30 oil for winter use. But I had always heard about protecting the yellow metal bearings in the transmission with the gl-1 oil.