Most synthetic oils are extremely thin in consistency, even if they have a high "hot" viscosity rating. They're still water-thin, no matter what the operating temperature of the oil happens to be. Stovebolt engines have minimum oil clearance specifications when freshly rebuilt that are beyond "worn out" numbers on modern engines. Most synthetics simply don't provide enough cushioning effect between parts to prevent noisy operation and possibly low oil pressure and damage from parts hammering each other due to the extreme thinness of the oil.
For instance, 5W-30 or similar viscosity synthetics simply don't have the physical thickness of the 15W-40 oil I use in every engine on my place, from Diesel farm tractors to cars, trucks, lawn mowers, welders, pressure washers, and a few other pieces of equipment with 4-stroke engines, even a Harley soft tail with an EVO engine. No oil-related problems in over 30 years, and it's a lot less expensive in 5-gallon pails from the local farm store!
Jerry
