10-40 got a bad rap back in the 80's when GM announced they would not honor the engine warranty on any engine using 10W-40 oil. When I questioned some field engineers and GM training center instructors about the policy, they were pretty evasive, especially when I asked if a specific brand of 10-40 was involved. Later, the real story came out- - - -certain brands of 10-40 had polymers in them to extend the viscosity range that had a tendency to coke at high temperature and form hard carbon deposits near the heat riser and EGR passages in the cylinder heads. That restricted pushrods and wore out camshafts, particularly on Chevy 305 engines. Rather than call out a specific manufacturer, GM simply issued a ban on all 10-40 oils.
15W-40 Diesel rated oil is an excellent choice for stovebolt engines, year-round.
Jerry
