It is not clear from your post if the problem is the lock mechanism itself or the gas cap.
If the lock mechanism itself is binding when the cap is in the open position, then I would inspect the lock for damage and clean it (you said you've already done that), then lubricated it with dry graphite lock lubricant (used by professionals). You can get it in most hardware stores or a locksmith. You only need a little, so buy the smallest tube available. The dry lubricant takes a bit of working in, so a few puffs in the key hole, then back and forth with the key. The fuel in the gas tank should not effect it (unless you overfill and submerse the lock mechanism).
However, if the lock operates freely when the cap is in the open position, then you have to look at the cap and locking tab for source of the problem. As the other posters indicate, you need to push down on the cap before turning the key to open it. If the cap or tank is bend, let say, or there is debris in along the seal, then it might prevent the cap from being depressed enough to prevent the binding.
Chris Glennon - Portland, OR