Yeah but if that were the case, then why not just put 0W in all cars? I'm not a fluid dynamics expert, but I would venture to guess that a thinner oil doesn't provide the protection necessary in an engine with looser clearances. Modern engines use thinner oils due to the tighter clearances. An engine with looser tolerances can be more reliable but less efficient and it is reasonable to believe a heavier oil would be needed in that case. It isn't always about flow. Even in my manual, it states not even to use 5W-30 in the 3.5 unless you're in a frigid climate, only 10W-30.
Just my $0.02 on that.
It is very difficult to tell over a video if it is indeed rod knock but if I had to guess, I don't think it is or at least it doesn't sound severe. It sounds like something else to me like bad compression in one cylinder.
5W-xx is much thicker when engine is cold than xW-30 is when the engine is at normal operating temp., so it should not be a problem, and again, at startup, you want the oil to get circulated to “dry” parts faster. As far as 0W-xx, I only make relatively minor deviations from the mfr’s recommendations on oil viscosity. That’s why I mentioned making adjustments “within reason”.
-synthetics, you didn’t want too much spread between the two numbers because to achieve wide spread, they had to load the base stock with much higher proportion of special chemicals (additives) which wore out quickly, and the hot and cold viscosity numbers were not maintained very well over the oil change interval.
With modern synthetics, that isn’t the case. The hot and cold viscosities are much more stable over the life of the oil with longer change interval, so there’s not as much reason not to go more with lower first number and higher second number. Everything is a compromise, and the greater stability of the viscosity numbers with synthetics gives more freedom to spread the gap between the numbers for better lubrication both at cold sartup and at normal operating temps. over the complete change interval. (I’m also conservative in my change intervals, because the higher stability with wider number spread only goes so far. IOW, where I used to do a 3000 to 4000 mile change interval, I now go with 5000 mi. change interval with a synthetic - none of this stretching it to 8k or 10k miles or more.)
It’s known that the recommended viscosities are a compromise between fuel mileage and wear, and they push it more to the fuel mileage side for more favorable federal mileage ratings. The consumer is educated to value fuel mileage as the only important thing, and generally doesn’t realize the compromise in the mfr’s recommendations. I’m willing to push it back more in favor of lower engine wear and longer engine life. 1/2 mpg or so loss of fuel mileage is not that important to me, whereas engine longevity is.
People should make their own decisions based on their own priorities with all the information at hand to make an informed decision.
As far as tighter tolerances of today’s engines, I used to think that was the case too, but if you look at clearances and tolerances of engines of 20 years ago compared to today, you’ll find that they really aren’t much different. Also, as an engine wears, those clearances open up a little, so that is an argument in addition to greater stability of modern synthetic oils (if you use a synthetic) for moving the second number slightly higher on a high-mileage engine. And, again with the stability of a synthetic, when you consider that the viscosity at cold startup even with a low first number (say, 5W) is much thicker than with a higher second number at normal operating temperature, there really is no reason not to move down to 5W (or 0W if you want to stretch things in that direction) with a good synthetic.
Those are my opinions, and that’s all they are, but they’re based on some solid info. and personal judgement calls. As the saying goes “YMMV” - literally.