I've re-greased two Vixen SP/GP mounts with great success.
Degrease with kero (get the low odour stuff if you're planning to do the work indoors)
Re-grease with a lithium based grease - used a BP brand I got from Supa Cheap Auto.
As already pointed out, the instructions at astronomyboy's website are great.
For the SP mounts, adjusting the backlash is a bugger of a job
