stock ones are sealed and are greased at the factory. thats why they last that long. you can rebuild the driveshaft with sealed ones and then not have to worry about greasing it again and then just rebuild it again in another 50,000 or so miles. but its the peace of mind being able to grease it at every oil change and not have to worry about it exploding that makes the greasable joints worth while.

I used the Precision ones. I am always surprised that when I grease them at 5000km interval they are always empty ?? Is my grease too thin ?? I use all-purpose grease.

I reused the original snaprings instead of the cheapy rings that came with the new joints.

Did you also grease it correctly when you rebuilt it? Or just did it at your next oil change.
When you did grease it did you get new grease coming from around all 4 seals on the u-joint?

Technically you're supposed to grease them before assembly, though I don't know how it affects the service life if you just do it after installation. I've done it both ways, depending on my mood and haven't seen a difference.

Missing a couple of maintenance greasings shouldn't cause failure so soon, though it will reduce the overall life. It's highly dependent on the environment they live in though.

As far as what brand you use, that's totally up to what you're comfortable with. But I've been using Neapco/PDQ for years without any issues.
The PDQ u-joints do have a warranty.

On the other hand, it could have been a defective u-joint out of the box. Any new part can turn out to be defective.
I once spent many hours trying to diagnose crappy running on my '62 when I swapped out the distributor, plugs and wires, thinking I'd screwed something up. Turned out 3 of the 4 brand new Champion plugs were bad. I still use Champion sometimes.

A good quality EP NGLI #2 is the correct grease.
I use Castrol Pyroplex Blue, but there are a lot of good greases out there.

I found that the grease that the factory packed in my new bearings had hardened to something like wax, I had to clean the old grease out of the X before I could get new grease pumped through.