I have used synthetic in my NH TC33DA since the first oil change. Mostly because I have drums of the stuff for my commercial use also. Even if I didn't, I would probably still use a synthetic, primarily since my NH probably racks up as many hours in winter as in summer. I only change the oil once a year, in the fall. Like Chris, I don't rack up a lot of hours, but it is only a gallon of oil in my TC33, so I don't see any real waste is using the synthetic in there. Does make one wonder, if they have a compact tractor, along with whatever implements, mowers, etc than have for it, that they would worry about cost in using a synthetic oil when it is only a small amount. If it was taking 10 gallons per oil change like my semi truck, then it could reasonably be argued that if one is not racking up a lot of hours, the synthetic might be a bit extravagant. Most folks have no problem dropping more on a weekend going out to eat than they would spend once a year to change the oil in their compact tractor, yet they want to be misers about what they put in their engine. Go figure.
As for the changing once a year, it may not be necessary from a oil breakdown due to engine time idea, but I also have concern over condensation and its associated issues from lack of every day use of the equipment also. I suppose that would only be a concern depending on what area of the country one lives. With the broader temperature extremes of the upper midwest, I feel better just changing out the oil once a year. Again, it is only a gallon of oil. Sure isn't going to break the bank account.