My model pneumatic ram is mostly some sort of 'plastic'. Head, tail and piston are 3D printed with PLA; cylinder and spear are PVC waterpipe; O-ring is Nitrile.

Because its function involved continuous extension and retraction I am looking for some type of liquid lubricant. Currently I am using Vaseline (petroleum jelly) as a grease but I think the lubrication needs to be on-going.

What type of lubricant should I use?