Sliding window

Mines rather stiff and despite keeping it nice and clean it takes far too much effort to use. Anyone had a similar problem and if so how did you remedy this. Is some form of luricant appropriate - if so what to use needs to be slippy but not wash off in a shower.
Ideas?

 

I know but I'm not telling you until you answer the question on your other thread

 

Silicone spray is good stuff.