My first car was a 1965 Chevy Corvair with an air cooled six cylinder 164 CID boxer style engine. I usually ran SAE 30 Quaker State API SE oils in that thing. While in college in upstate NY I routinely started it at temperatures below zero F, and sometimes at minus 20 F. I would say to just start the tractor, warm it up or use it and then change the oil.
Has it occurred to you that maybe a 15w-40 or 5w-40 diesel oil might work for you all year round?