I keep my atvs my cabin, so i don't ride them that much. I change the oil once a year. i usually change it in the spring. would it be better to change it in the fall so they don't sit all winter with the old oil? or would to continue what im doing and change oil in the spring so that the new oil doesn't get moisture in it?

 

i change my oil acording to the owners manual.
and thats every 300 miles.
if you dont ride it in the winter then just do what you do change it in the spring befor you start riding it.
as long as you change acordingly it should be ok.