My main gripe about using the ultra-thin oils like 5 or 10 weight is the fact that these engines were not built with the hair-splitting super close tolerances of modern engines and they need the cushioning effect of thicker oils. No matter what the upper weight rating happens to be, a cold engine is getting pounded apart when it's trying to lube itself with oil the consistency of water. I use 15W-40 in everything I own, from lawn mowers, to cars, to motorcycles, to tractors. It's a good compromise between cold start flow and cushioning, and hot, high speed lubrication capability. The engines run quieter, and wear less.
Jerry
