Seems to me that only aftermarket replacement u-joints even have a zerk fitting, and I'll bet the mfr. would recommend grease.
As you know, bearings don't use grease, they use the oil in the grease...90-wt. would certainly lubricate, but I think there would be some leakage. I agree that the hard part would be getting it in there!
The secret, if it is one, is re-greasing regularly, and not over-greasing. I do it every oil change, while I'm under there...just a half-squeeze with a hand grease gun.
I use #2 grease (aluminum complex with molybdenum), Bel-Ray I believe, for everything.
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