When i came across these type of plug valve for the mod, i'm sure i remember ordering Audco grease, a real thick gunky grease.
IF you contact a company like Flowserve, they'd probably find you a more cost effective alternative valve and advise where to get the grease.
Let us know what you find.
By the way, i haven't actually worked out are these toilets ok for number ones and number twos ? I wouldn't want to chance a number 2, so what's the normal protocol when out at sea?