They aren't allowed to under CAFE, if your vehicle's CAFE rating is done on a 0W-8, that's the only grade you can spec. That's why the grade charts vanished from manuals.
Why do bearings knock? It's because the clearances have opened up to the point that they make noise, it's not a function of oil pressure, oil pressure is an artifact of resistance to flow, when you have bearings that are worn out, the wider clearances allow more leakage which means less resistance to flow which means lower oil pressure. It's like the difference in running 20W-50 vs 5W-20, you are going to see WAY more oil pressure with the 20W-50 (perhaps to the point of putting the pump on the relief) because it leaks less due to being heavier, which creates more system backpressure, which you see as oil pressure on the gauge.
Put some much thinner oil in those come bearings and see how long they last. In journal bearings, a thinner oil will result in a smaller MOFT, which if you do the calculation of the shear rate inside the MOFT it actually creates more shear rate than if the MOFT is thicker. More shear rate also means more temperature increase, so going thinner isn't saving much temperature increase overall. Like I said in a previous post, a YT guy who tracked his car a lot though he would just run the 5W-20, and after a few track sessions he inspected the rod bearings and they were extremely worn. The oil temps might have been 10F lower, but the MOFT broke down to cause excessive bearing wear. If he would have ran a thicker oil they would have survived better.
Maybe someone has the factory service manual so they could look it up. The main reason they survive on 0W-8 or 0W-16 isn't because they have tighter clearance, it's probably because they are wider and have more surface area to support the loads. If they are too tight, they will over heat and destroy themselves. Journal bearings being too tight is a killer since the MOFT temperature skyrockets and the viscosity in the MOFT goes so low that the bearing starts destroying itself.
Look at the bearing clearances on the XSR900 I posted in that thread I linked to in an earlier post. Here's the snip:
You missed the premise of my post. I was saying that a journal bearing does no rely on the oil pressure to support itself. All it needs is an adequate supply of oil to at least make up for the side leakage. Many journal bearings in industry have been simply gravity fed, like this.
Feed oil pressure does not "support" journal bearings. How is 50 PSI gauge at the inlet of the journal bearing going to create 10 times that pressure simply due to the supply pressure? Do some more self study on journal bearings - there's endless info on the 'net. As I said above, even if the oil is simply supplied under gravity, the natural action of the rotating journal bearing draws in the oil and creates the supporting wedge which could have a pressure of thousands of PSI in the MOFT. The oil pressure you see on the dash is simply the resistance seen from the PD pump trying to force oil volume to all parts of the oiling system. If an adequate supply of oil volume is delivered to moving parts, then there is adequate lubrication as long as the viscosity is adequate enough. You could have adequate supply oil volume, but if the viscosity is too thin, it could cause problems. There needs to be both adequate supply volume and viscosity.
I was writing if the oil pressure is one psi with perfect clearances. Not clearance makes it one psi. Maybe the engineers can try it out on their engines.
0psi works fine with gravity lubed systems. You seem to have missed the point
This is all true for most bearings, but not for rod bearings. Rod bearings have a minimum oil pressure requirement to receive any flow at all. This is because the oiling supply is within the rotating crankshaft, and is affected by centrifugal forces. This minimum pressure increases quadratically with engine speed, and when oil pressure drops below this value, flow is completely interrupted, and MOFT drops to zero.
Of course it may not work that well for rod bearings on reciprocating rods on a crankshaft. Again, the whole point (and context) of my example of that journal bearings can operate on just gravity fed oil supply was to emphasize that the oil film wedge that puts journal bearing in hydrodynamic lubrication and supports them is not due to the oil pressure, as someone thought. What actually supports them is simply the oil wedge that naturally forms inside a rotating bearing as long as the oil supply is getting inside the bearing. And plenty of journal bearings all around the world in industry have been simply gravity fed an oil supply. The premise is: Get oil into the journal bearing and it will create the supporting oil wedge if the rotational speed and viscosity is adequate enough to do so.
I'd think the viscosity being too low has more to do with it. Keep in mind that the lower the MOFT goes due to thinner oil, the higher the shear rate becomes in the MOFT wedge, and the higher the wedge temperature becomes due to the shearing factor. That somewhat cancels out the heat that would be generated from shearing a thicker oil with a larger MOFT (less shear rate), and why the temperature rise isn't that much different between the viscosity used. When the dust settles, thicker oil is going to always give more MOFT with all other factors being constant.
But it's been shown many times that the same engines used in different countries do put other oil viscosities in the owner's manual. Snap shots of the OMs have been posted in almost every thread like this (see link below - tight bearing clearance is not "designed" just for thin oil). As pointed out earlier, tighter bearings can tolerate thicker oil, while looser bearings can't tolerate thinner oil as well.
There are facts, and there are misconceptions. BITOG has pretty much been about separating the misconceptions from the facts.
Are you making stuff up with every post? Because you certainly are not listening to nor comprehending any of the technical information that is being presented in this thread.
It absolutely matters that CAFE is the reason. If you can't spec other viscosities, you only get the one the vehicle was qualified on, it's a pretty simple concept.
But they DO put thicker oils in the manual in markets that don't have CAFE, like Australia. Just like @ZeeOSix posted, here are the oil spec pages for the 2023 Corolla from Australia:
Toyota, IF….., more data needed.
