I recently switched to this grease from Amsoil.
Synthetic Heavy-Duty Grease, NLGI #2, GC/LB (GHD)
I was using Mobil 1 red grease, but didn't like how it ran out of the grease gun. I used it for many years w/o problems, but the Amsoil product seems better. Grease applications depend on how the vehicle is used. If it vehicle is used off road, in mud etc, follow Shannow's suggestion. In normal PC use, I grease mine 3 times/year.
The Amsoil GHD is a very good grease for this application, but under most circumstances, any grease with the GC/LB classification will work just fine.
When I had my Jeep, I greased it every time I changed the oil, which was every 5-6K.
Anytime you wheel in deep water or mud, you should grease all your u-joints and steering/suspension. If you don't wheel and don't have a lift, then they really don't need too much maintenance. I'd say check'n them or adding a pump or two every 5-6K is good advice.
I use a little tub of Pennzoil EP grease. It was $2 on clearance from WalMart.
I just pulled one of the original u-joints off my Jeep at 170K, and while it was loose, after pulling the caps it still had plenty of grease after all this time. Replaced it with another non-serviceable u-joint, but I think it might already be going bad after just 6K.
With that Zerk fitting, you now have a hole for the water to get into, or grease to leak out.
But like Zaedok said, rough water conditions [immersion] would make me give them a squirt of grease.
And as long as you have them, the 5,000 mile thing is OK too.
Keep in mind the logistics of greasing this micromotion application. You don't want to use too thick of a grease. If too thick, it'll get squeezed out instead of flowing back in to the contact area. Kluberplex BEM 34-132 is well-suited for this application. It is an NLGI 1.5 grease made for micro-motion applications, resistant to elevated temperatures and has good washout resistance.
My 2008 4Runner has them and interval is 15,000 miles. I use Valvoline durablend moly grease. Toyota doesn't want you using a high moly grease in ujoints and specs lithium based grease. The Valvoline is a lithium based grease with only 1% moly so you get both and it is safe for ujoints as well.