My understanding is that clearances between mating parts are much tighter in current automotive engines, compared to what was normal in the past. This requires a lower viscosity oil to get between mating parts, which also reduces friction and improves fuel consumption. Tighter piston to cylinder clearance allows less rocking motion of the pistons, especially when cold, and allows more consistent ring contact with the cylinder walls to reduce blow-by into the crankcase. When I was in high school, I worked at a full service gas station part-time. There was a regular customer that had a 1966 or 1967 model Chevelle with a 325 hp 396 cu. in. engine who would change his oil only when the hydraulic valve lifters started clattering. We would change the oil and filter and, by the time he left, the lifters were quiet as new. This was a regular occurrence, and when he stopped for gas it would still be quiet for a good while, then it would start clattering again. Apparently, the viscosity modifiers in the multi-weight oil would break down, and the oil would stay at its lowest rated viscosity when the engine heated up to normal running temperature, which was too thin to keep the lifters pumped up. This is an extreme example because most people change their oil often enough that it doesn't reach the point that the oil viscosity fails to increase at higher temperature. But it does show that it is important to change multi-weight oil at least occasionally for this reason, and because other additives like detergents, acid neutralizers, anti-foamers, etc., even in straight weight oil, get depleted over time with use.
I do not think you understand viscosity indexes
The first number is measured at 25 C the second one is between 80 & 120 C depending which standard you use.
All oils get THINNER as they get hotter
They do not and can not get thicker
What multigrades do is GET THINNER SLOWER
People conflate multigrade "thickening" with observed oil burning & sludging .
As for the lifters, they were responding to the fact that the particulates in the oil slow down it's movement and prevent the oil filling the lifters.
In extreme cases the particulates in the oil force the filter bypass to open so you are in a low oil pressure situation
This is exactly the same as running with low oil.
In any case it has zero to do with mower engines apart from Kohler Commands as mower engines are strait solid lifter systems .
It is a simple experiment I used to make all of my students on their first prac session .
Get a big funnel and put an obstruction in the outlet ( which is the actual tool that is used ) so the outlet is no bigger than 1/8" diameter
Get a tall container and mark 2 lines on it about 1/2" from the bottom & top
Fill the funnel with cold oil & measure the time it takes to drain from the lower line to the higher line
Tip the oil out and heat it in a pan ( outside on the BBQ if you want to sleep in your own bed the following night ) to say 100 C then tip it into the funnel again & measure the time again.
It used to blow the students minds and we had groups or 2 or 3 so they could test a lot of different oils ( and other fluids ) .
By 1985 I was no longer allowed to do this prac session unless it was done in a fume cupboard, all of the participants had to be wearing full fire suits & respirators which is a shame because it was probably the best value session for generating deep discussion as every one had been exposed too BS advertising so they all "knew" what the results would be and were dumbfounded when the expected results were not obtained .
