saginaw transmission

I have just purchased a race car and it has a saginaw 3 speed transmission. According to research it is supposed to have 80w90 gear oil  GL4 in it. Are there any recommendations from experienced dirt track racers on what I should use? Ive heard to use ATF because it is thinner and takes less power to move it therefore producing more power to the wheels. 
Also I have only found GL5 80w90 would that be ok to use?
thanks
Jeremy