i talked to my old man whos an engineer at nasa and he was telling me that basicly (this is the dumbed down version) grease for gears and oil for bearrings in this situation. he said that the downfall is that grease atracts airborne contaminants if not sealed which could be an issue with cross contaminantion. he went on to say that if engineered correctly, there should be more "space" left around the piston for grease than there would be for oil due to its viscosity. this small amount of play due to the oil pushing the grease out is enough cause chatter. he then instructed me to remove the piston clean it completely of the oil and then use a regular marine grade grease. christian @ inkmachines.com instructed the use of "any normal grease". i used the marine grease and it worked like a charm.dont forget all matter in the universe is trying to make its way back to its original form, with this said petrol based products will break down anything else with petrol in it.