I have a 1997 Ski Doo MXZ 440 liquid cooled and gas is getting in the oil tank. How and why?

[Jeriann]

I have a 1997 Ski Doo MXZ 440 liquid cooled and gas is getting in the oil tank. I have put about 1500-2000 miles on the sled and have not had to add oil to it (tank is always full). Oil is light bluish green. My brother figured gas is getting in the oil tank because of the smell of the oil. What could be causing this problem?
Sled is also hard starting. Is this part of the oil/gas problem?