shear stable oils - good or bad?

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.
 
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Sure. They practically rely on the shearing down of VI improvers to help with fuel economy. However - most engines are tolerant of a wide range of motor oil viscosity, even if the manufacturer doesn't state it. They have to cover their collective asses given that there are shear-stable oils out there. If a carmaker says 5W-30, they have little control over whether or not an individual owner uses a shear-stable one on the thick end or one on the thin end that will shear into a 5W-20. It's probably not a matter of what the engine will properly function on. It's a matter that most people (i.e. conventional oil users) will hit the "sweet spot" of decent fuel economy and reduced warranty claims.
 
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rememember 5/20 is a fuel economy oil NOT the best oil for a IC engine CAFE maks the mfg recomend it but it is a high wear oil in my mind I like 15/40 bruce
 

medic

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quote:
Originally posted by bruce381: rememember 5/20 is a fuel economy oil NOT the best oil for a IC engine CAFE maks the mfg recomend it but it is a high wear oil in my mind I like 15/40 bruce
With many of the current oils being very shear stable, I am not totally sure that 5w-20 is a bad oil and I would at no time say it is 100% NOT the best oil for some IC engines. I have to agree with Dr. Haas, which puts me in pretty good company.
 
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Also the car manufactures know their vehicles are operated mostly at minimal power levels. If you read Dr. Haas' writings he doesn't operate anny of his vehicles at very high power levels,a drag race iw WOT 12 seconds. Get Dr.Haas to come and visit me and we will go to Nevada and I will get his motor hot enough to prove 20 wt. oil has its limitations. In fact I have a doctor friend in Ozona Fla. who could do it better than I, with a smile all the way.
 
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quote:
Originally posted by bruce381: rememember 5/20 is a fuel economy oil NOT the best oil for a IC engine CAFE maks the mfg recomend it but it is a high wear oil in my mind I like 15/40 bruce
But isn't he saying that 5W30 = 5W20? I think he's saying there's no difference between the two in an operating environment because of shearing of the one and lack of shearing of the other. I took his point to be that the changing standard of oil grade recommendation does not actually represent a change, rather an acknowledgement of the changing reality of oil quality. Is that not what was meant? I certainly agree with Steve S regarding usage factors.
 

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Yup, that's pretty much the point I was trying to make. People who were running 5w-30 oils 15 years ago actually had 5w-20's in their engine the majority of the time due to the oil shearing down very quickly. Today, many of these same people flat out refuse to use a quality 5w-20. Actually, what fueled this thread was that I am considering running GC in my Harley Davidson. This engine typically shears a dino 20w-50 down to a 40 wt in no time and sometimes down into the 30 wt range. So, I am probably gonna run a thick 0w-30 that is 100% shear stable and see what happens.
 
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