Just bought a 2020 brute force 750 and I want to service it as recommended. I'm ready to change the break in oil out of the front differential but I'm getting different answers. The book says to use API GL-5 Hypoid Gear oil either SAE 90 or SAE 80. Online trying to figure out if that is correct, many are saying it's a miss-print in the Manual. I questioned it because I have always used 10W40 in my previous brutes. Any help is greatly appreciated. Also, synthetic or not?