The Stovebolt Site Forums The Shop Area The Electrical Bay Timing question.

The 20 in 20w 50 is how it flows at cold temps. "w" = weather. The 50 is how well it flows at operating temps. The oil you are using is too high in both areas. Studies show gas mileage loss from 1% to as high as %12. Also, your oil being high viscous (thick) does not flow in cold or hot temps. This may be good for an older motor but not good for mileage. You can't have both unless maybe synthetic.
Oil companies say use the truck manufacturers recommended oil. That may not be true for our older trucks built in an era before oil was perfected. Before dual viscosity. Way before modern bearings and gears and such. The newer trucks recommend around 5w30/5w40.