My diagnosis is the extra fluid blew a rear seal and now you've got transhydraulic fluid leaking on the (supposed to be) dry pto clutches.
My Cockshutt/Oliver 550 that I bought this spring suffers from exactly the same problem. Probably not coincidentally, the fellow I bought it from had not long before had the 3-point hydraulics worked on. I got it home, piddled around with it for a day or so and then hooked it to the bush hog. Engaged the PTO - no problem. Slip clutch on mower worked right and no racket anywhere so I started out. I made one round in 2nd gear, stopped to change to third, and bingo - no pto output.
Plus I think I've got some engine oil seal leakage too. Although I haven't opened the inspection plates to see, I can see small drips of darker engine oil on the front plate (guessing this is for the driveline clutch) and clear transhydro drips at the rear. I've never tried to split a tractor before but this might be the time to learn.
Reading your situation I see some similarities: start out cold and relatively drip dry and PTO engages and things seem to work OK (but probably not perfectly), stop to do something or change gears after the oils heat up and you get hot oil in more volume on the clutches and not implement action.
Can't wait to see what the real tractor mechanics and long time ollie owners have to say about it. -WSJ