You have to figure out where your problem is before you can fix it. Changing the heat range of the plugs will only mask the problem, not fix it. Check the main jets in the carbs and compare it to the factory specifications for your sled for temperature and elevation. Then check the oil pump to see if it is indexed correctly, as jearl suggested above.
If it's fouling at wide open because the fuel/air mixture is too rich, changing to a smaller main jet will help. If it's fouling when the power is in the midrange, you can lean out the midrange by changing the position of the e-clip on the jet needle by removing the needle and raising the e-clip (lowering the needle) one notch. If it's oil fouling, adjusting the pump like jearl suggested is the ticket.
If you do any jetting changes, make sure to pull the spark plugs and check them often. Lean is hot and hot will burn holes in the pistons! The plugs should be a cardboard brown when the jetting is spot on.