I had a few gallons of fuel mixed up by a buddy a few years ago. It has been stored in a cool, dark environment. After rebuilding a helicopter a couple months back I started to use some of this fuel. I couldn't figure out why the engine was getting hotter then what I had remembered ( haven't flown a helicopter in 25 years ). Last time out I flew 1/2 tank and then left the heli on a startup bench. After an hour or so I noticed an alarming amount of condensation at the top of the fuel tank. The fuel has absorbed enough moisture to cause the engine to overheat. In spite of this the engine started, transitioned well and seemed to run " Just Fine " except a little down on power and getting hotter then I thought it should. Grabbed a new gallon of 15% and it runs much better then it did with the old 30% my buddy mixed up for me. Now I have to force myself to dump out 3 gallons.