I don’t understand why the kids have only metal or teflon washers to chose from. The teflon is expensive and the metal is heavy (not to mention not very slippery). Is there any reason why Vex can’t supply something like a nylon washer? Nylon washers would be much cheaper than teflon.
I know nylon can absorb moisture and change its dimensions somewhat, but the Vex spacers are made of nylon, I think. Is nylon too fragile, maybe? 
You can use the thin nylon spacers, but in some cases those are still too thick.
I don’t believe the “Vex teflon” washers are really PTFE. They don’t have that Teflon feel to them. So you already do have nylon washers at the price of Teflon ones! 
I’m sure they’re not nylon though and they probably are PTFE in some form… just not the usual stark white you’re used to seeing.
There’s an official Q&A entry asking about permitting washers along with screws and nuts. Hopefully the GDC will allow washers next season.
So do any adults want to put this to the test? PTFE contains a lot of fluorine, but Nylon, as far as I know, has none (unless it’s added to brighten the plastic or something). Anybody want to set up a spectroscope and flame test the washers? Fluorine ought to give off a very distinctive spectral line.
But don’t breathe the fumes!
Okay, I’ve just learned it’s not a good idea to flame test fluorine-containing compounds like Teflon (PTFE) because they can generate toxic gases. So don’t try this at home. 