We always use to use "black bear" grease on splines at my last heavy equipment job. The stuff is thick and dosnt move much. I wouldn't use normal grease as it can attract dirt wearing your splines more.
Honestly - keeping it clean and greased with something is more important than what type of grease. Even the very best grease will fail given enough time. I don't think there a product that is - "grease once and forget".