I don't disagree with the "pump until you see new grease" thought, but only under certain circumstances. If you've got the truck on a lift and the slip yoke is extended, there's a lot of room in that cavity to pump in grease. If the truck on the ground, then the slip yoke is already partly...
You're fine to mix synth and dino. I think there's a zerk on each u-joint and one near the slip joint. Somewhere I read that people just pump grease in until you see it coming out the splines. But contrary to that I've also heard that you shouldn't do that because if you compress the DS...