Slickoleum Light Grease
Slickoleum Light Grease is the king of all mountain bike seal greases. Mechanics use it for fork seals, shock seals, dropper post seals and even as a freehub grease.
Fun fact: SRAM use slickoleum to manufacture their very similar product: SRAM butter. But that costs about $20 for an ounce. Why not get the original at a better price?