I have a question reguarding old fuel myself. not unopened fuel that has been stored properly but, fuel that has been opened and say carried over the winter. When I open a new jug of fuel I put my pump on it and don't change it until that jug is gone. That being said, the last two years I have had to replace bearings in two engines An OS .91 FX and an OS 46 FX. I fly during the winter when weather permits, usually 3 or 4 times a month. On each occasion the engines started running poorly on the old fuel but would run fine on new fuel. Being somwhat tight wadded I didn't discard the old fuel but chose to burn it up. My question is could the old fuel accumilate enough moisture to cause the bearings to rust but still run in the engine. I always run the engine dry after a session and add a little air tool oil for ARO. I'm sure I didn't experiance a lean run. There was rust on the bearinga and crank shaft. My shop is not climate controled unless I'm in it, so the stored fuel goes through temp changes. In the spring and summer fuel don't last long enough to get water in it.