no same fill level. It is the same unit for four wheel as two wheel drive. as far as I know they still all bolt on the the output side of the tranny or transfer case. Therefore can only be used in two wheel drive. unless they have came out with a new model since I last looked. oh by the way you are not supposed to use the gear vendors units while towing or engine braking. If you do this will void your warranty. A friend of mine found out the hard wy when his unit failed. After he installed a new unit he found out that U.S. gear makes a overdrive/ underdrive units that can be used while in 4 wheel drive, towing or engine braking. we both agreed that this pretty much makes a gear vendors unit worthless on a working truck.
as far as oil goes just follow the manufacturers viscocity and remember that synthetic is usually rated for higher oil change intervals, heat and wear resistance. we used amsoil gear oil and followed the operating instructions given to us by gear vendors and the second unit never failed for the next couple of years that he owned the truck.