I also use 10w40 Heavy Duty Castrol in my transmissions, but I'm in a very warm (65F-74F last week) climate. Transmission fluid is something not changed every year or even two, it is viable for 500 hours, it is good for many years. That said, I might consider using a lighter weight oil in cooler climates, unless I was going to warm the engine at 1/2 to 2/3 throttle for several minutes and/or use the machine a severe duty situation for an hour or more, snow ploughing, furrow poughing, etc. The heavier oil in cooler climates will take longer to get the transmission moving and working. To tell the truth I do not know which scenario is best, but with engine oil the right viscosity for the right climate is deemed important, but that may not be the case with transmission oil, or it might be. In the northern states they are using ATF, due to sub zero weather, but as the tractor warmed up, or was under a greater strain over an extended time. I am not sure this choice would be good, but again, I'm not sure as transmissions and engines behave differently. Good Luck