I have what I think is a pretty good theory about why some motor oils are noisy.
First, in the 1980 -1990's I was using a 5W-30 syn-blend in my 87 Jetta (1.8l gas). Back then the syn-blends were Group I and PAO (with probably some esters mixed with the PAO). There is a road near where I live that winds down into the valley that has a 3 ft concrete divider about 1ft from the left lane. If you are driving on the road with your drivers window open, you can hear what type of noise your engine is making because the sound reflects off the concrete barrier. In the 1990's I was surprised how my engine made quite a "racket". It almost sounded like an engine with 400k miles on it instead of 40k miles. With the current Group II 5W-30's I now use, my engine is much quieter.
There are still some skeptics about this, but I think we've had just way too many accounts of noiser engines with certain oils to discount this effect, and I've experienced it myself. So what is going on here? I think the high noise factor of some oils has to do with the PAO/esters content in some of the lower viscosity grade synthetic motor oils.
One of the significant differences between Group II and PAO's oils is the pressure-viscosity coefficient, or the (CT) coefficient of traction -- basically how slippery an oil is in a sliding pressure regime. PAO's have a much lower CT than Group II mineral oils -- they're slipperier. But oil film thickness is also related to the CT of an oil. The higher the CT, the thicker the oil film. If we compared a 6.0 cSt GII and PAO, the GII will actually produce a thicker oil film in a high pressure sliding regime.
Quote:
Lubrication Technology.pdf … Film thickness is a function of viscosity, pressure, temperature, relative velocity, surface roughness, and other factors. All else being equal – geometry, velocity, load, and viscosity- some base oils will provide a thinner oil film. Mineral oils will provide the thickest film, followed by PAO, PAG, and esters [3, 5].
Also …
Machine Lube " …A high coefficient of traction correlates well with thicker film formation, even under extreme and variable conditions. …"
If we list the CT of
various oils, Group II have the highest CT and will produce the thickest oil film (GII is even higher than GI) …
Group II … … … … highest CT thickest oil film
Group I
Group III
PAO … … … … lowest CT thinnest oil film
There is also a good graph on page 7 of this article comparing the base oils …
ZDDP Tech Brief "…when subjected to extreme pressure, oil's viscosity will increase as its molecules are forced into closer proximity to each other. Under extreme pressure, oil with a higher 'α' value (pressure viscosity coefficient CT)
will have a thicker film due to the resulting viscosity increase…."
I should point out, and as the Lub Tech article relates, a mineral oil may not have the advantage of a thicker oil film over a PAO all the time. If we heat the 6.0 cSt GII and PAO oils up way past 100C, the Group II oil will thin more than the PAO because it has a lower VI. At some higher temperature the increased thinning of the GII may actually end up having a thinner oil film than the PAO because the viscosity has thinned much more than the PAO. But for practical purposes, I think we can assume that an equivalent GII will in most normal temperature instances produce a thicker oil film than a PAO.
Another factor to consider in the GII vs PAO oil film thickness are VII's (viscosity index improvers) and the fact that PAO's have better cold flow properties, so one can use a thicker base oil to make the POA based motor oil with less VII's. I'm going to assume that VII's are not really "that" slippery and will have a CT even much higher than the GII. So the addition of more VII's to a conventional based 10W-30 will raise the CT of the GII base oil even more than the PAO oil, leading to an even bigger gap in the CT for the GII oil over the PAO. For example, using some ballpark numbers, a 10W-30 GII based motor oil might start with 5.5 cSt base oils and a PAO might start around 8.0. We would add 2% VII's for the PAO and much more VII's (around 5%) for the GII to get them both to 11.0 cSt (30 weight range).
Since the 11.0 cSt blend of GII + VII's will likely have a higher CT advantage than the PAO + VII belnd than they originally started out with, this will help the GII based oil produce even a thicker oil film. These thicker oil films will buffer and quiet the metal/metal slapping of the camshaft, and perhaps other engine components like the bearings, which leads to a quieter engine. One might also consider that the big molecules of the VII's acting like coil springs, so they may provide some additional type of buffering effect where 2 metal surfaces are trying to contact each other.
Most of the noise complaints about oil that I've seen on this forum have to do with the thinner Mobil 1 oils like 0W-20, 5W-20 and 5W-30. Since these oils contain a healthy dose of PAO/esters, this probably contributes to a thinner oil film and more noise than the conventional GII based 5W-30's, or even the GIII based 5W-30's. I've heard people comment that when they stepped up to a 40 weight Mobil 1, the noise went away substantially, so even the higher viscosity PAO/ester based oils can get to a point where the film becomes thick enough for noise to no longer be an issue. This noise factor may also somewhat depend on the engine type, camshaft design, and bearing clearances of the particular engine -- in other words, some engines may be noisier with these synthetic oils, and some may not see much difference.
In summary, a high CT (coefficient of traction) Group II oil with a good dose of VII's will produce a high CT oil that will produce thicker oil films than oils that contain Group IV/V synthetics. In turn, some of the lower viscosity grade motor oils that contain a good portion of PAO/esters, produce thinner oil films that are capable of producing noisier engines. The thicker oils films produced by the GII oils in combination with VII's in many cases can provide better buffering of the engine metal/metal slapping noises, producing a quieter engine.
