I had an HP service tech out yesterday to replace the "pizza wheel" assembly (yes, some marks cropped up after I said they weren't there, but by the time the tech came, the marks were gone on the print, but he replaced the wheel assembly with the updated one anyway).
Anyway, he was telling how important it is to keep the steel transport bar (the chrome bar the ink head slides on) in the Z printers oiled up. He said putting a few drops of oil on the bar every few weeks will greatly enhance the printers life. He said the friction from the bar over time loads onto the other components causing unnecessary wear/tear. He also said that if the bar isn't properly lubed, it can cause some degradation of print quality because the carrier can't slide smoothy.
This is the HP part # he gave me for the oil: 6040-0854
I asked him if this was in the manual, and he chuckled and said "no."
Just fyi.