Has it been knocking all the time or did it start this winter? Reason being, in some parts of the country gas is reformulated during the winter due to air pollution and I noticed on another one of my vehicles that I had to move to mid-grade during the winter due to a slight knock under load. Also, as an engine ages and builds up deposits in the combustion chamber the compression increases slightly which could affect the fuel requirement. I alternate adding mid and premium in my 1988 245A because I can tell the difference in power and it needs all the help it can get. Don't think it gets better mileage, but definitely has more spunk when accelerating onto the freeway. I use Chevron with Tectron and have always passed California's strict emissions test.