I second the Rotella T-6 5w40. I've really learned to appreciate it this past winter, and I'm sure the tractor does too. I think Delo makes a 5w40 as well. Either would be an excellent choice for your area in Canada.
The winters get cold, -10 to -30C. This year we have a pile of snow so I'll take your advise and go synthetic. It is not synthetic now is there any issues switching?