As said, any grease will do if there's no slippage. "Carbon grease" or "carbon prep" is grease with some grit mixed into it to keep a seat post from slipping. The reason carbon seat posts slip more than metal seat posts is because with carbon seat posts, you can tighten the clamp only up to a relatively mild clamping force.Use carbon grease specificly for carbon to any material. Richey sells carbon paste in little packets, tacx sells it, etc.
And that is the ONLY reason to use carbon assembly paste - when things slip if you dont. I've used just grease with CF bars and posts with no issues.I have an aluminum stem/handlebar that tend to slip with plain grease, but ever since the use of carbon paste, no problem. I've even been using it with aluminum seatposts and steel frames lately.
That little extra bite allows me to go a little easier on the wrench.