It is your tractor you can put what ever you want in it. However like was posted already the book says SAE90 in summer and SAE80 in winter. It also says MINERAL oil. Gear oil isn't the same thing. I would talk to an oil supplier and see what they recommend. Oils have changed over the years since the manual was printed. If you went to a Massey dealer now what would they recommend? Don't use engine oil in it, they are not the same thing.