Aeroshell or Phillips Aviation Grease No.22 or No.6 seem to work well.
The only glider manual I've seen which actually says anything specific
about grease is the one for the L33. I think it gave a Mil Spec
equivalent to Aeroshell Grease No.6. If you use either of these wear
gloves when you're applying it. It's a lot harder to wash off than the
white lithium!