ATF can be corrosive to metals and certain rubber seals. The transmissions that use it are designed for its properties. Chain cases are more similar to differentials with 75w-90.
But I agree with BC_DAN. It really doesn't matter. If you run vegatable cooking oil, you will get the exact same life as someone with $$$ factory oil. (as long as the ATF doesn't eat your seals and leak out at least, that would be the * in the story.)
1) 2004 Yamaha Warrior
2) 2006 Yamaha Nytro