I can't imagine the Schaeffer's would be much if any cheaper than the Polaris oil, but I certainly wouldn't hesitate to use it. They have a 5W-50 that would work great too. I usually used either Rotella T6 or Amsoil's 0W-40 Powersports oil in my Polaris. I haven't been on the ATV forums in a while, but the most popular choice always seemed to be M1 0W-40. Some guys on there had some pretty high miles on their wheelers. I know there was a guy who had over 10,000 miles on his Polaris using M1 0W-40.
Edit...I missed the part about you already having the Schaeffer's on hand. In that case, I'd definitely use it since you already have it.