brass or oil impregnated bronze. My money would be on the bronze. It's not about the price either. Less than $6 compared to $13+.

At least you have the options in this thread with pics.

I've had good luck with the stock plastic pieces - available at your local Datsun/Nissan dealership for just a couple bucks each. I'm on my first set of replacements, so I really can't complain about having to replace them too often in the 40 years I've owned the Z.

Dennis