Before I put any sprays on it (in some cases I have found silicone to bind, not lube), I would clean it out. It may be swelled with dirt!
The way the sliders work, there is a track fitted inside the frame. When water runs down the two windows
, it either runs outside the track (on the outer window) or into the frame, under the track (inner window), taking dirt with it. There are usually two little hoods sticking out from the bottom outside of the frame. These cover drains for the frame.
Get some stiff material like heavy wire or weed-whacker monofilament cord and shove it in and out of the hood drains (you can pull the hoods right off if you wish). Then carefully squirt some water with hose (some will leak inside, so don 't overdo it) in the hood holes, then more shoving, etc.
Note that the track material may go up and around the bottom corners before it ends.
I found after my trip on Yukon Terr highways with the calcium chloride that the stuf almost turned into a cement and took wire to get it out.