Black Tank Gate Valve Leak

For some reason the black tank was always harder to open and leaked before the gray. When I replaced mine I thought it was tricky to replace and had to do it twice because I had a defective valve. I did not want to do this job again so I installed a grease fitting on the top of the valve a use my chassis grease gun to lubricate once a year or so when ever it starts to feel hard to open. The grey valve is the original(20 years old) and still does not leak the black was replaced 10 years ago with grease fittings added to both.