Molybdenum disulfide, also known as "Moly" type grease. Barnone the best shear strength/wear resistance. Or, if you are concerned with water, get some of that waterproof grease.

Actually, any decent grease will work.

Tip: double check that you tighten everything down. Also, be sure to take all the slack out of the steering-head bearings, just to the point that it doesnt make the steering bind. Any slack at all will make the front chatter and may cause all sorts of horror. You really dont want to discover these problems at high speed.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
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