MMO, I used it to soften the seals in a tractor lift. Got the hydra fluid warm, added a gallon MMO and worked the tractor for about 30 min. It has about a drop a day compared to a quart a day. Good stuff. Engine restore...well...you'd be better off spending your money on a rebuild kit.
Another vote for Marvel Mystery Oil. Great stuff for certain applications. It has a very high flash point, as jamsxr found out he could heat the jug.
I used it years ago on a few conveyor chain applications, but that was before I was skewled reel gud on what the proper oils for that were. :D
It was designed to be a top end lubricant. Perfect for the application spanky stated.