While a granite surface plate is a precision item and deserves good care, it's also useful to remember it's a very hard wear-resistant rock that's been around for millions of years, so don't be too paranoid about it.
Starrett (and others) sell special surface plate cleaner. I've got a quart that has lasted me for years. But, I suspect any mild cleaner would be satisfactory if you don't want to bother getting the "official" stuff.
Try to make a living, not a killing. -- Utah Phillips
Don't believe everything you know. -- Bumper sticker
Everybody is ignorant, only on different subjects. -- Will Rogers
Law of Logical Argument - Anything is possible if you don't know what you are talking about.