Manufacturer's suggested oils for that tractor will be WAY out of date. In my old Gravely, I put synthetic oil in it. After about the 3rd change, it was no longer using oil.
Most modern synthetic is a 0w30 Mobil 1. This is what I'm using in all my engines now, even the 1983 Benz diesel. It's manual calls for 10w40. That was the best of multi grade oils available in 1983.
The lower-numbered (e.g. the "0" in the 0w30) values will give the engine better protection at start because the oil pump will pick it up immediately. ALL oils will be heavier at startup than the last number (e.g. the "30" in the 0w30) in their weight designation.