I bought 1 x 200 ml can of PTFE spray from e-bay for about £5.00.
It was enough to do both my 18 foot poles twice. So one 200 ml can will do one 18 foot pole four to six times (I was probably a bit liberal with the spray).
I'm now using furniture polish, which doesn't seem to be as good, but the saving on PTFE spray will be enough to fund replacements; I'm sure.