"Thin oil can't create pressure" is an oversimplification. It depends on what is being pushed through where.
Very, very simply, pressure is force applied per unit area. The formula is Pressure = Force / Area
If you put your entire body weight on a large sofa, it'll exert less pressure than trying to sit on a bicycle seat. Trying to put your entire body weight on a sewing needle will exert much more pressure, enough to piece the skin.
Does it make sense that it's more difficult to suck honey through a straw than water? This seems fairly obvious. It's because honey is thicker than, or more viscous than water. For the purposes of this explanation, you can assume that it takes more force to move more viscous things.
Now, based on the formula, how much oil pressure you have depends entirely on:
- The thickness of the oil being pumped, i.e. how much force it takes to pump
- The area through which it's being pumped, i.e. bearings, oil galleries, oil pump, filters, etc. Basically the entire lubrication system.
The important thing is that if an engine specifies 5W-20 oil, the engineers designed the oiling system to have sufficient pressure and work perfectly with that oil under the specified conditions. The flow volume of the oil pump, the flow volume in the bearings, fuel economy vs. reliability, everything is thought of when that recommendation is made.
If an engine specifies 15W-40 oil, and you pour 0W-8 oil in, the oiling system won't have enough pressure because the gaps in the system are too large to create a useful film of oil for moving components to ride on and not touch each other.
Similarly, if an engine specifies 0W-16 oil, 20W-50 oil will be too thick to pump, and if the oil pump can pump it at all, the pressure will be much higher than specified, and components that rely on oil flow for cooling won't get fresh oil in time.
The consequences of too high or too low pressure are similar, but only in that the components won't be lubricated properly. You're generally fine to go a grade higher, but going lower than the minimum specified viscosity can really damage things.
Basically, follow the manufacturer's recommendations in the owner's manual. Yes, Toyota has new engines that recommend 0W-8 under warranty.
