Hi folks, thanks for all the helpful replies (very belated I know.... just sitting around watching the snow fall...)
Just wanted to post what I used as my solution:
I used Synco's "Super Lube High Temperature, Extreme Pressure Grease"
The "Synthetic Multi-Purpose Grease" was definitely too light!
The mast rotated like a charm afterwards.
Here are pictures of the cup & pin that the mast sits on, for those that have a similar setup:
First pic is without the boot; the second one is with the boot.
I'm not sure what the cup is made out of.
I don't have a photo
of the "male" side of the rotating hardware
handy, but it is a convex stainless fitting that is affixed to the mast end cap.