Try this little experiment before you put a water-thin oil in a stovebolt engine. Put a pan of oil on the stove, and use a candy thermometer to check the temperature. Tell me how thick it gets when it reaches 250 degrees. Now tell me if you want a liquid the consistency of water trying to take up the clearances between pistons, rods, and other moving parts. The engine is going to hammer itself to death.
Your money- - - - -your choice, but it's very simple to cut through all the bovine scatology that gets posted here with a real-life experiment.
Jerry