I've been reading threads recently on oils, notably Valvoline vs. Mobil and 0w-40 vs. 10w-30. Then I came across this article on motor oils:
All About Motor Oil
After reading through it, I have a question for the fluids gurus on here (F&L GoR?). Specifically, I am wondering about the amount of Viscosity Index Improvers in 30 weight oil as opposed to 40 weight oils. This statement bothered me: "...first, these molecules are not lubricants, so the more of them that you add the less oil you have sitting around lubricating things." If 10w-40 oil has enough VIIs in it to increase the viscosity by a factor of 4 and 10w-30 has enough to increase the viscosity by 2 1/2 (and I assume 0w-40 would have even more VIIs) as the author states, am I better off using 10w-30 rather than 0w-40? Or is this insignificant in a Viper engine as compared to a motorcycle engine that also routes the crankcase oil through the transmission?
Just curious...
I disagree with the statement that the VI improvers are not lubricating things.
Hydrodynamic lubrication (i.e. when the oil acts like a fluid and metal parts "surf" over each other - this includes crank bearings and other places where you could measure oil pressure) doesn't really care what the fluid consists of. Whether it is base oil or VI improvers, the metal parts don't come close to making contact, so the important characteristic is simply whether the fluid can keep from squishing out too quickly. This is pretty much a definition of viscosity and VI improvers obviously help. It's the same as surfing - you don't care what is dissolved in the water as long as you don't hit rocks under the water.
A single grade has no VI improver. A mineral oil 5W30 may have 8% while a synthetic 5W30 may have 5%. A mineral oil 10W30 may have 6% and a synthetic 10W30 may have 2%. A mineral 10W40 (maybe the mineral oil with the largest VI improver amount) can have 10%. A synthetic 5W40 or 10W50 can have more than 10%. None of this is linear; the graphs of viscosity are logarithmic so I don't like the use of 2.5 times or 4 times the viscosity.
VI improvers do break, but the engine and bench tests required for the API categories disallow any tremendous decrease or increase in viscosity. The distance the metal parts are kept apart is dependent on the fluid viscosity, so maintaining viscosity is important. But the change in viscosity actually doesn't change the distance as much as thought. How many times have you used one grade higher or lower in an engine? In that experiment, you have changed the viscosity more than an oil will shear down or thicken up due to use. Also, running the oil another 20F-30F hotter will also change the viscosity.
The reason motorcycles have different requirements is because the transmission and engine share the same oil. The gears will shear VI improvers. There are different chemistries of VI improvers that would work in transmissions, but there is no way the consumer would know what's in the bottle, so OEMs simply recommend against multigrades. Yes, you are correct, this is the main reason engines and motorcycle use may require different oils.
Additives are surface active molecules that under the heat and pressure of imminent metal to metal contact, partially decompose and form a bond to the metal. Friction modifiers, anti-wear additives, and some detergents behave this way. This is boundary lubrication and while it sounds bad, it happens slowly over the 100,000 miles of engine use. The amount of these additives is the same in single grades with no VI improver and in 10W40 with the most VII. Both provide equal boundary protection so there is no debit with the multigrade.
Summary -
VII do provide hydrodynamic lubrication
Having VIIs in the oil does not decrease boundary lubrication
Viscosity change during use is smaller than changing to a different viscosity grade oil.
Synthetics have less VII than mineral oil because the base oil has VII-like properties
Motorcycles have transmissions that shear VII, so motorcycle OEMs recommend synthetics
So it's not that it is insignificant, it is more accurate to say it doesn't apply.
