I have a 13 and a 16 and they both do it. 13 is worse. Check valves only keep oil going forward so no drainback so if it is siphoning new valves won't fix it. Elevate front of sled in storage so it goes into engine, convert to premix or drain tank down for storage. Also do you have an aftermarket plastic oil tank (BDX, C3, TKI)? Both of mine do with belt drives and problem got worse with them after installing as the tank sits higher in regards to oil pump compared to stock causing more gravity feed. My 13 has a 3 qt tank and is worse than my 16 with a 2 qt tank. Added boost to the 13 so now I don't see a leak and the excess oil gets forced into engine. No need to fog for storage. LOL