Hello BF2, It sounds like a city bike? That stays outdoors year round. I.ve never had this problem. I'd get a (Spray) can of lithium grease. The only place that sells it is around here is Auto Zone. Give it a good spray. Usually WD-40 is pretty good for your outdoor house locks, padlocks, etc.
A few Home Depot's around here. Sell auto parts, fluids, etc. Located at the front wall of the store. WD-40, Stands for water displacement. But it is useful on bikes. New Index shifting systems, seem to work better, with slightly more maintence. 10-4