Welcome back to the smoking section Smithdad...
I'm happy to see you and your son have not given up racing / riding but have found a less expensive alternative, not to mention a bike that will put a bigger smile on his face... Yes there is a "re-learning" curve, but I'm sure he will make the transition rather quickly. Agreed the lack of engine braking is "different", but once he re-learns how to carve and steer with the back wheel, he will forget about the engine brake... Not to mention, I'll bet he's having more fun then he's had in a while. Isn't that why we race / ride, for the grin factor. I'll also bet your wallet is going THANK YOU... THANK YOU... THANK YOU....
Yes you are correct, the choice of which oil to run can be very personal... You say there are no good MX shops in your area. I'd stop and see what they carry, and then base your decisions on that. As far as my bike(s), I run Amsoil syn premix, and AFT in the tranny. If been using AFT for a loooong time with zero issues. I've been using Amsoil as my premix of choice since its carried locally in my MX shops, and usually can be found at most MX shops.
I run Rotella T in my cruiser, and all my 4 stroke equipment. My equipment doesn't see the abuse a MX bike receives so I am 100% sold on it.
Again welcome to the smoking section... Now just sit back and enjoy the ride.