Does 5w-20 flow better than 5w-30 when colder than 40 deg C? Seems like having the same -w rating would imply they both flow the same. Factor in the usual positive displacement oil pump. Thus, one could argue the 20 weight is primarily a CAFE issue if fuel economy testing is on a warm engine.
In the older days the evil viscosity index improvers were enough to keep you away from larger spread grades. You could believe then that 5w-30 or 10w-30 was better than 10w-40. I don't think this is still the case.
After seeing that Ford failed CAFE looking back 3 years, and they make most of their money on trucks, I am skeptical of their motivations. You can imagine an internal meeting with a bunch of business managers and engineers thinking, hmmm, how do we keep making a high proportion of trucks so that our profits are fat given CAFE? Put smaller engines in them? Nah, consumer won't buy 'em. If we up our corporate average .05 mpg then we can sell 100000 more trucks in the mix. Decision made.
In the older days the evil viscosity index improvers were enough to keep you away from larger spread grades. You could believe then that 5w-30 or 10w-30 was better than 10w-40. I don't think this is still the case.
After seeing that Ford failed CAFE looking back 3 years, and they make most of their money on trucks, I am skeptical of their motivations. You can imagine an internal meeting with a bunch of business managers and engineers thinking, hmmm, how do we keep making a high proportion of trucks so that our profits are fat given CAFE? Put smaller engines in them? Nah, consumer won't buy 'em. If we up our corporate average .05 mpg then we can sell 100000 more trucks in the mix. Decision made.