Something I posted on another thread but it seems to be a popular issue.

The engines we use seal the bearing by liquid lock. If you want a big term for it "surface tension" (The a property of the surface of a liquid that causes it to behave as an elastic sheet). The tollerance between the crank and the case is critical to this working which is why loosing a bearing can make a engine non rebuildable. It is common for boat and some plane engines to not even have a seal at all. Our engines have a seal to keep dirt out of the bearing not seal vacuum. If you use after run oil or leave fuel in the crank case it will leak out the bearing over time. If the engine runs fine I wouldn't loose any sleep over it.