I have been thinking about this for a while now and finally decided to ask for others opinions. Many 5w-30 oils shear down to a thick 20 wt oil in no time at all. Motorcycles can shear a 10w-40 or even a 20w-50 down to a 30 wt pretty quickly also. Common sense tells me that engineers knew that this was going to happen when they designed the engines. Common sense also tells me that they considred this when they recommended a certain weight of oil. Such as with Ford and Honda. Then, with the newer technology oils that became more shear stable, they changed to a 5w-20, knowing that this is what they wanted in the engines originally. So, assuming that I am correct, if every oil was as shear stable as GC, in most cases, we would be able to drop at least one grade from what we are currently using and be better off. With this in mind, running an extremely shear stable oil, such as GC, in an engine that calls for a 5w-30 because the engineers knew it would shear to a 20 wt, may be a bad thing. You end up with a thick 30 wt oil in an engine that was designed to have a thick 20 wt oil. At the same time, running a thick, shear stable 30 wt in an air cooled engine that calls for a 10w-40 or 20w-50 may actually be a good thing. Just a thought.