I never said the shaft was softer, just that it might not be hardened steel (i.e. unsuitable for linear ball bearings), and any metal on metal contact will wear out both parts. It just so happens that the brass/bronze gets worn down more than the steel. As for the lubrication, not quite. Polymer bushings will lubricate as long as the bushing is there, because the plastic itself is the lubricant. Bronze/brass bushings (the sintered kind) are impregneated, basically soak in and absorb oil like a sponge through all the pores left by the sintering process. This does run out, and while the plain brass/bronze will slide relatively well on steel once the oil has left the bushing, I think most people would recommend adding oil. I totally agree on the tolerances, and I mentioned the same thing in my earlier post.

I simply recommended polymer bushings because they are (slightly) lower maintenance. The original poster (at least from what I've gathered) is just looking for advice to get the machine back up and running to do its job as quickly and easily as possible. For much the same reason I wouldn't recommend replacing the shafts and going for linear ball bearings.