I agree with John. Silicone grease works great for this problem, and the bushings will last longer besides.
You will need to detach the brackets and slide the bushings to one side to properly apply. Clean the surface of the bar with brake cleaning solvent (or something that does not leave a residue) before applying the grease. I also had to use a little sandpaper to get the surface smooth and clean.
The end bushings actually take the most abuse, since they allow for some 'slide' as the control arms move in addition to the 'twisting' motion you would expect. After 200K miles, the metal in this 'sliding' area has actually worn down to a noticable extent in my case.
Most other (non-MB) cars have an additional short 'end-link' segment to absorb lateral motion.
I have been searching for replacement polyurethane bushings with no success. If I do find them (or fabricate some), I will fit them with a zerk fitting so that they can be greased without dissassembly. Poly bushings are somewhat more likely to groan or squeak if not greased then rubber.