I had a Jonsered 2141, and now a 2152 that both leaked. I couldn't find the source (I have to admit I didn't look very hard). I did ask the dealer where they were purchased, and he said "it's just the nature of some of these saws". Got a used Husky 357XP and it never leaks. A friend with another 357 leaks as bad as my Jonsereds.
Alsayyed's post has me wondering if I may have messed up the gasket between the two halves of my case. I pulled some bolts from the front of the saw to replace the dogs shortly after I bought it. The oil tank may have been pressurized at the time, due to temperature changes. Wonder if I blew the gasket out at that time?