20W40 is still available from Ingersoll dealers but it's about 10 bucks a quart. That grade used to be popular 30 years ago but not today.
You can use 15W40 all year round providing your tractor is not exposed to temps falling close to zero F when it is sitting. Put the trans-axle in neutral, start the engine, push the travel lever into full forward and leave running for 10 minute or so at half throttle to warm up the oil.
If you live in an area where the tractor will not be used in temps lower than the freezing point, then 20W50 is a better choice.
If you live where the winter temps are almost always near zero F, then you might want to consider using 0W30 synthetic. However, as soon as the temps go back up above the freezing point, then switch back to 20W50 Dino oil.