Mineral oil is used as a laxative, but you take a couple of CCs to begin to produce that effect. The body basically can't process it at all, and it just lubricates its way through. Hence it is safe to eat. Though I guess if you drank the stuff constantly you'd have problems with dehydration because your large intestine would not have any time to reabsorb water.

Those looking to protect their food processing knives a bit might look into bee's wax.