Chaincase in those does not use ATF Fluid from factory. They now call for using Full Synthetic chaincase oil from Klotz, Polaris or AMSOIL. Yeah you can use ATF it but you will be changing it EVERY year then vs the proper oil from Klotz or Amsoil or Polaris. Only snowmobiles I ever saw call for ATF from factory was our old Yamaha.
For the injecting oil just use Polaris Blue or something similar as Dan said.