Which grease to use when servicing RST suspension fork I would really need an answer because my forks got really stiff and I want to service them.
The user manual says "Do not use Lithium grease"  

Slick Honey, Slickoleum, SRAM Butter or any other branded bicycle fork specific grease/butter is ideal. Although expensive, you purchase and use such small quantities its probably not worth worrying too much about looking for alternates. 

Alternates if you want to go down that path would include any motorcycle fork oil seal lubricant. A general purpose automotive grease will probably dissolve in the fork oil so is not particularly ideal, but will probably work fine if you service your forks regularly (I would not use it)