I don't have a tractor yet, but I have worked on engines and gearboxes for years. I was also an engineer at Cummins where we built engines. I can tell you that starting an engine does indeed cause wear. Running an engine does indeed cause wear. Working an engine does indeed cause wear. Idling an engine does indeed cause wear. What prevents wear? Oil. Starting your engine, any engine creates heat, turning your engine off lets the heat dissipate. Take any item, heat it, cool it and you create condensation. Condensation will mix with the oil and reduce its' ability to prevent wear. Run the engine long enough to get it up to operating temperature UNDER LOAD for at least 15 - 20 minutes and you will burn off the condensation that you created last time you heated the engine up and cooled it back down, start it up and don't get it warmed up and work it, you won't. These are not opinions but well documented facts. With all that being said, I don't start my motorcycle just to warm it up in the winter. I get on it and ride it across town at 80+ MPH for 30+ minutes to get to work. I have put hundreds of thousands of miles on motorcycle engines, taken them apart, and put them back together, transmissions (or any gearboxes) are the same way. That's why they have the little breather caps on them, to let the condensation out!! Use a good oil (I prefer synthetic) and keep it changed with a new filter, THAT'S the BEST thing you can do for your engine!! Now, does that mean that the OP has ruined his engine? Probably not, but it could. Your engine, take the chance if you want, but now you can't just shrug your shoulders and say "I didn't know"...