There was a thread on here last week covering this same thing, that is of course if it is in fact leaking from the cap. Sorry couldn't find the link to it.
My warranty is voided due to me buying my sled in the states, I faced the same problem. Try wrapping the threads of the oil tank with plumbers tape ( Silicone tape ) in a clockwise direction. Others were making a tin foil cap to go under the lid, but over the threads, they said that worked for them.
If the oil tanks cracked thats a different story. 4 out 10 of my buddies XP rentals have had the oil tank replaced due to someone putting their knee into it and them cracking. Just something to be aware of, if it happens it does but I still wouldn't trade my XP for anything else!!