Regarding the article that was posted.
While the writer may be a chemist he failed to leave out a few Engineering facts. He stated that thicker motor oil provides the best protection. (20w50 is better than 10w40 in a hot climate).
One must remember that a high revving inline 4 has its oil supply system designed much differently.
A lugging V-twin has much larger oil supply system than your typical inline 4 and can handle the thicker oil more efficiently. Bearing tolerances are also much larger on something like a V-twin and require a thicker volume of oil to fill up the void between surfaces. Sort of the reason also that V-twins can use hydraulic lifters.
A high revving inline 4 has smaller oil supply lines but nature of the thing requires that more oil be circulated faster. Bearing tolerances are much closer so a thinner layer of oil between the moving parts is required (hence 10w40 is required here & not 20w50) Because of the much faster internal parts movement, inline 4's can't use hydraulic lifters (you would not be able to keep them pumped up fast enough at 12,000 rpm) You would not get suitable oil circulation in an fast revving inline 4 with oil too thick.
This is why racers will use the thinnest oil possible. (But again, they don't require long life out of their engines)
On the original question of 10w40 versus 20w40.
To make 10w40 versus 20w40 in regular oil, the manufacturer must add more Polymer’s to the base stock. This in turn makes the oil easier to break down.
In synthetic oils this is not so much of a difference as the base stock is much more consistent, and it is much easier to design weights.
So (and this is a big if)
Is 20W40 than 10w40? Most engine wear occurs on start-up so you have a double edged sword here. The 10w40 would be much better preventing wear at start-up. The 20w40 would prevent wear at peak operating temperatures (depending on the day). However this is all thrown out the window, because you would now have to depend on the flashing rates of each oil brand, Polymer breakdown, etc. etc.
I use motorcycle specific synthetic 10w40, and I use the brand that has the best breakdown results in independent tests. (I stress independent tests, and not the ones that were paid for by the said oil company. I don't use Amsoil for this reason)
