Intersting you should ask. I'm getting ready to winterize my 2015 Supra, and came across the same question of fluid. The manual says it requires Mobil Delvac Synthetic 50. I had no idea what that was, and got ahold of Larry at Indmar. He said this is just a normal synthetic oil normally used in manual transmissions. It's SAE 50 weight, so anything similar will work just fine.

He also said that if the fluid looks clean, on the dipstick, you shouldn't need to replace it. He said it should last at least a few years. I figure if mine looks good, I am not going to mess with it.