I have bikes with Shimano and a Sram dérailleur shifters with the same problem. The dérailleur spring is not strong enough to return the arm to the two last high range gears. The highest gear being the position where the dérailleur's cable is on least tension and the spring is at rest. I know that this is probably a lubrication problem rather than a spring problem so what's the best way to fix the problem? i.e., what lubrication, and where do I apply it?