There’s plenty of information bandied around about which lubricant to use, and the fact that the service manual only specifies lithium grease only serves to confuse the issue. This applies to the CX500, GL500, CX650 and GL650.

Honda’s Moly 60 paste, or something with a similar moly content. Moly – molybdenum disulfide to use it’s full name – is an excellent extreme pressure lubricant, even when it appears to be dried out. Many of the more easily acquired moly greases only feature 3-10% moly content – not enough for this application. This paste is commonly used on both Honda and BMW shaft drive splines, and is no longer available like it used to be sadly! Links removed.
2015 update: This Moly is apparently now NLA, though it is still available from many sources – in the meantime try the Honda alternative part 08798-9010 (eBay.