I bet you'de be hard pressed to find anything but trace elements of
teflon on an impeller after it has pumped water for an hour or so.
Lubricate if it makes you feel better but it really only makes assembly
a little easier. Beware what you use though, cause a lub with the wrong
solvents in it can damage it.

I agree with preventative maintenance being important because impeller
pumps are a cheap substitute for a better positive displacement pump.
The tolerances are not nearly in the micro range. That's why even with
the wide range of dimensions that comes from the plastic molded ones
driven by a drill, they still perform pretty well until they completely
wear out.

Ron