Chiming in a bit late on this lively discussion..
I've always used the Bel-Ray Saltwater proof grease on all motorcycle and moped steering & wheel bearings. It's not a synthetic, but it does hold up very well.
The Phil's stuff is fairly thin compared to the Bel-Ray, and for some reason it doesn't impart a lot of confidence. Moped and motorcycle wheels do higher RPMs than bicycle wheels, and I'd be concerned about the thinner grease flinging out.
And in the steering head, it never hurts to have a good stiff grease.. adds a little damping action, and that is never a bad thing.
On a bicycle, you want to eliminate even the slighest friction or drag on any of the moving components, so the thin Phil's stuff makes sense. But it's still overpriced.