OEM's changed to lighter oil to meet the unrealistic fuel mileage and emission regulations imposed on them, plus the almost nonexistent clearances in modern CNC-manufactured engines are so tight that thicker oils won't fit. What we once considered to be normal oil clearance is "worn out" in today's engines. Running water-thin oil in an engine that measures oil clearances in feet instead of inches is foolish and non-productive.
The major disadvantage to running a very heavy oil is lack of lubrication at startup, especially in very cold weather, as well as increased load on the battery and starter due to the high cranking amperage of turning an engine with oil the consistency of molasses in it. 15W40 flows like a 15 weight when cold, and lubricates like a 40 weight when hot, without compromising seals and other potential leak points in the older engines. Sounds like a win/win situation to me!
Jerry
