You can use a dry lube on those parts. If you use any oil or grease, they attract and hold the belt dust and gum up the works. Dry lube will stay dry and not attract dust, dirt and debris. It is the same stuff that is painted or sprayed on the garter spring assembly and outer bell of a new clutch.
I don't have a link handy for it but someone else may be able to provide a link for it.
Danford1