If your owner's manual doesn't recommend 5W-30 but it does recommend 10W-30, there is a reason regarding multigrade oils few people know.
It's called temporary oil shear. This is different and much less understood than than permanent oil shear that most people on BITOG are highly familiar with, which is the irreversible viscosity loss experienced in
used oil due to the permanent breakdown of viscosity-index improver (VII) polymer molecules.
Temporary shear on the other hand is reversible. However, it still reduces the operational viscosity, even for
new, unused oil. To understand this examine the following plot.
Source:
https://www.oronite.com/paratone/shearrates.aspx
Shear rate means how fast and how close the sliding or rotating parts are. It's given by the relative speed of the motion divided by the separation between the two sliding parts. Therefore, the resulting unit is inverse time (inverse second). Shear rate is highest in the valvetrain because the parts are virtually making metal-to-metal contact, with only atomic-level separation.
Temporary viscosity loss -- temporary shear -- increases with shear rate. Therefore, it's highest in the valvetrain. I don't know how accurate the Chevron/Oronite plot is but you can see it there. The plot is showing the viscosity contribution by the VII, not the total viscosity. Also, they don't show the timing chain, which could have even a higher shear rate. I think it's safest to assume that in certain parts of the engine, VII has no effect on viscosity-generated protection against wear. So, to be on the safe side, assume that your timing chain is seeing SAE 5W, not SAE 30, when you're running SAE 5W-30 etc.
As a result a 5W-30 runs thinner in the valvetrain and timing chain than a 10W-30, even when the oil is fresh. This is because a 5W-30 has more VII than a 10W-30. A 0W-30 would run even thinner.
If you want the highest wear protection, consider the thickest base oil you can afford in your climate. Base-oil thickness is given by the x in xW-y. a 15W-40 has a thicker base oil than a 10W-40; a 10W-40 has a thicker base oil than a 5W-40, and a 5W-40 has a thicker base oil than a 0W-40. Therefore, you would experience the lowest operational viscosity in the valvetrain and timing chain with a 0W-40.
Nissan engineers recently published a paper that claims the timing-chain wear is related to the base-oil viscosity, not the finished-oil viscosity that also includes the contribution from the VII:
Do you think that's "thick" oil? Think again!
As a rule of thumb, say you have a SAE 5W-30 oil:
* You're running a SAE 30 oil only at the normal operating temperature
and in the leak paths (behind rings, past seals, etc.).
* Otherwise, you're running a SAE 5W oil in the high-shear parts of the engine, such as the valvetrain, timing chain, and even the cylinders and bearings to some extent. Even if the shear rate in a given part of the engine is not extreme, you're nowhere near a SAE 30.
Regarding conventional vs. synthetic, the same rules still apply. Don't count on replacing a conventional 10W-30 recommendation with a synthetic 5W-30. Even a synthetic 5W-30 is made of a substantially thinner base oil and a lot of VII; so, it still runs thinner than a conventional 10W-30 in the high-shear parts of the engine. Besides these days the line between conventional and synthetic is highly blurred.
So, did you think you knew how a multigrade worked? Unlike most people believe and most sources on the Internet explain, a 5W-30 and a 10W-30 are
quite different oils, even at normal operating temperature, where they are supposed to have the same viscosimetric properties -- they simply don't at all! This is the untold fact of the multigrade oil.
