Well, I have no idea if this is suitable for your application, but I do know that clock's mainspring lubricants are as good as it gets. They can handle 300.000 lb/in2, they're very thin and one drop can cover incredibly large surfaces. I use one of the Keystone brand.
I've also heard of silicon nitride bearings made by Saint Gobain, called CERBEC, allegedly with very low friction coefficients. You'll have to look for them, as I'm in a hurry.
Of course, the lowest friction coefficient you can get is if you can make (I doubt it very much) the car to ride on air bearings. We had a thread where we mentioned them (actually, on spherical wheels): forum/viewtopic.php?t=2781
In this same thread we mentioned SKF magnetic bearings, which are used in satellites because of their extremely low friction coefficients.
Finally, I also mentioned in that thread the extremely clever Halbach array, which could
give you a way to build a zero friction (or so I imagine) bearing with no electric energy input.