Quick question what is the difference between gear oil’s

Shouldn’t have to add any, the only way it could be getting low enough to add is if it’s leaking, in which case repairs are in order. Transmission gear oil is usually, but not always lighter than axle gear oil. I have read that many manufacturers tried to run ATF in the manual transmissions in the ‘70s and it did not work out in their favor at all. Frequency of changing gear oil depends on severity of usage, so once again, the factory service manual is your best friend.