BelRay Gear Saver. Never tried anything else, but have heard of lots of different permutations.
I can see running motor oil in a crank case on a 4 stroke if the oil also lubricated the crank, but on a two stroke, if ATF, chicken and noodle soup, or what ever else one could think of was the right stuff, you would think that your local MX shop would be stocking little white and red cans, or whatever the magic elixer of the day was.
For the few bucks extra for the oil designed for the application, compared to the cost of lunching (hey maybe that's what the soup is for??) a tranny, I'll stick with the right stuff. As a note, once I switch to steel plates on the clutch pack, I have never had a problem with a tranny. This includes the time my son lost his oil plug (brought the fresh motor straight from a shop to the race site the morning of the race, and prepped the bike really early in the morning before start time) on loop one at a National in Utah, and ran the second 40 miles without any oil. Cleaned the tranny with solvent, let her dry, added some BelRay, and is still running great to this day, and that was more than 5 years ago.
Now if I could only figure out how to get a BPJ sandwich through that little hole, I'm sure my bike would go faster...
Rick