We had loads of experience w/ that when we took Elise's M109 up the eastern sierras last summer.
We started w/ a fairly new, to us, tranny. Couldn't get it to stop puking out the bellhousing. Ended up pulling the tranny right before the trip and finding that the oil return hole on the front had been plugged w/ some permatex from over use by the prev mechanic. Cleaned the hole out, and left a few days later.
First day was couple hundred miles at 2400. The tranny was hot enough to expand the oil and anything over it's desired fill level was pushed out through the bellhousing.
Elise was learning to drive the truck, climbing mountains, and the clutch would start slipping. We'd find a place to pull over, I'd roll under the truck w/ a can of brake cleaner (carried two, or three at all time) and spay it up though the drain hole (w/ a sheet of cardboard on the ground). She would work the clutch a few times, and off we'd go.
After a few days of driving the fluid level was at the desired level (for this tranny), and the clutch quit slipping.
Elise has now driven the "She Beast" over 3000 miles in the last year, about half on dirt tracks, and has become one He!! of a driver.