Can someone tell me what the problem is with shearing, or dropping out of grade, such as Castrol GTX seems to do, as long as the wear numbers are good? I'm afraid I don't understand too much about the subject.
If the wear numbers are good, then it's not a big problem, but keep in mind that when an oil thins out, it means it's VII is breaking down, so in the long run that engine will probably get dirtier faster. Although with Auto-rx available to come to the rescue, this isn't as big a problem as it used to be.
Hydrodynamic film thickness and viscosity are directly related. As one drops so does the other. SO if your oil shears down so does you hydrodynamic film strength. If you put high loads or cintrifical forces on the bearing and you oil has sheared down you risk bearing damage. The oils resistance to being squessed out from between parts under load is going to drop as the viscosity drops. The thinner the oil the more often you will need to take advantage of the AW additives as you will make contact more often if you load the engine.
Wear may be ok...but the problem is the shearing is indicative of other things. Such as high content of VII. They are not as stable so you WILL have a thinner film and I imagine they are more prove to coking up or leaving deposits.
I would really like to see a HTHS test before and after ~3k or so when an oil has sheared. Also compared to an oil that does not shear. I'd be willing to bet money the the sheared sample has lost HTHS stability.
The HTHS is just a quick bench test, it doesn't take into account long term stability. I doubt any permanent shearing takes place during the test. BUT the HTHS values can indicate the posibility of that, if you look at other variables.
Is loss of stability important for a reasonably driven daily driver...no probably not, as joeee12 points out. Push it to the limits or do some road racing and you may wish you chose a more stable oil...
Yes they usually do thicken back up after ~5k or so...but viscosity does not mean everything...what do you think caused the thickening.
Also a case for sampling at multiple points when first trying an oil...to see what it is really doing.