Grease is really not a wise rather a stupid choice to use on belt as the whole concept os you dont mess with the belts grip.
Wd-40 is not a good choice BUT WD-40 ""SILICONE SPRAY" is the only thing which should be used on all rubber parts including belts....sometimes in winters they just get hard so shots of this spray resolves this.
Last year i figured out the choon choon sound from new cultus suspension too which people were facing. It was basically thr upper front shocks mount whose rubber get hard in winter and make choon choon sound on breakersm a single shot of spray on top of both mounts resolved this issue.
"I learned this the hard way as suzuki changed the front control arms twice on my demand but still this problem rose in the same winter"
So yeah...WD-40 SILICONE SPRAY is the only wise choice....it has also been repeatedly recommended by Scotty kilmer.