I would suggest not lubricating any kind of plastic bearing. Dust will stick to the lubricant and increase wear. Depending on the lubricant used the plastic may also start swelling. You can easily make thinks a lot worse by wrong lubrication. Keeping the bearing clean and the pushroad polished is probably the best approach. If you want to use a lubricant then maybe a thin film of something on a silicon or teflon basis would be good. In any case make sure the lube is compatible to the sleeve material and will not cause swelling.