After 650 miles, I too am noticing a small amount of sqeeking over big bumps on my Duster.....I will be applying the grease over the winter.
I see other products on the market, ......
a green synthetic marine grease and Super Lube 41150
[ame="http://www.amazon.com/Super-Lube-41150-Synthetic-Cartridge/dp/B0042FG74W"]Super Lube 41150 Synthetic Grease (NLGI 2), 14.1 oz Cartridge: Power Tool Lubricants: Amazon.com: Industrial & Scientific[/ame]
but since Energy Suspension makes the bushings, I will use their grease