Thanks, StevieC for posting this. I just read the short article in the original post. The title is very interesting.
Can you actually make up for lower oil viscosity by higher base-oil quality so that you actually have more protection against wear with a thinner oil than you would with a thicker oil of lower base-oil quality? This is the million-dollar question. Some here will jump and say, no way, viscosity is what lubricates! Unfortunately, it's not that easy. Why? Because viscosity is not a single number. It's something that not only depends on the temperature but also load pressure. In fact, it gets even more complicated because modern oils are multigrade and use viscosity-index improvers (VIIs) to increase the viscosity. But how do VIIs react when pushed to their limits?
Consideration A:
One interesting comment in the article is about the oil-film strength and how 0W-16 oils require the best base oils. Indeed, Mobil 1 0W-16 is PAO-based and Mobil Super 0W-16 is GTL-based. As far as I remember, TGMO 0W-16 is the same as Mobil Super 0W-16. So, no more Group III with 0W-16, at least so far.
Oil-film strength means the pressure - viscosity coefficient (PVC). It means how much the oil film can be squeezed under the load pressure before metal-to-metal contact occurs, in other words the failure of the elastohydrodynamic lubrication (EHL) and start of the boundary lubrication.
The tricky part with the PVC is that it's strongly temperature-dependent, decreasing sharply with the temperature. Group I mineral oils have much higher PVC but do they still stay high at high temperatures? Synthetic oils have much lower PVC but do they have the advantage of being much less affected by the temperature?
This Chevron patent studied how PVC changes with the temperature and found that the cam wear is directly related to the PVC100/PVC40 ratio. The closer the ratio of PVC at 100 C and PVC at 40 C is to 1, the less cam wear they observed. The GTL base oils had a PVC100/PVC40 ratio much closer to 1 than Group II oils and offered much lower cam wear, even with thinner oil.
Chevron patent on decreasing wear using GTL with PVC100/PVC40 close to one
Consideration B:
Viscosity index (VI) is an important factor in determining if the oil will stay thick when the temperatures keep rising. When pushed to their limits, bearing temperatures can easily exceed 150 C where HTHSV is measured and reported. In addition, there is still the ambiguity caused by the viscosity-index improvers (VIIs). Is it more important to have a base oil with high VI or is it OK to increase the VI using VIIs?
Consideration C:
Better the base oil (PAO, GTL, etc.), less the VIIs you need. But what do VIIs do?
This paper by Timken found out that when pushed hard, VIIs may phase-transition from liquid to solid, which in turn prevents the formation of an oil film because oil cannot pass through a solid. The result is wear because there is no oil film.
High-pressure viscosity and tribology of lubricants with viscosity-modifier additives
Babak LotfizadehDehkordi, P. J. Shiller, K. K. Mistry, and G. L. Doll
Timken Engineered Surfaces Laboratories (TESL), The University of Akron, Akron OH
May 2016
(
link)
PE is the polyethylene VII:
Table 1 presents that measured film thickness for PAO base oil is in agreement with estimated film thickness using Hamrock-Dowson equation. However for PAO/PE B blend by increasing contact load film thickness collapsed. This collapse correlates with the pressure-temperature sharp increase in viscosity of the specimens at 75 °C. It is also observed that after the WAM test at 75 °C of the PAO/PE mixture specimen, the fluid transformed to a semi-solid state. It is noteworthy that PAO/PEB blend demonstrated similar film thickness to the PAO and estimated film thickness using H-D equation at 40 and 100 °C.
It is not clear how the lubricants perform in the contact zone after VM phase transitions, or how the solidified VM passes through the contact, or how it functions if it does pass through the contact. However, it is likely that if VMs undergo a liquid-solid phase transition in the inlet region of the contact, the solidified material will jam or block the flow of lubricant, causing a collapse of the lubricant film. This scenario is consistent with the wear of the glass disc (Cr-coated), used in the EHL measurements. It is concluded that the plate wear arises from a collapse of the films in the contact zone. Furthermore this collapse correlates with the pressure-temperature sharp increase in viscosity of the specimens at 75 °C.
My take:
Even when leaving the additive package aside, viscosity (grade) alone is not the answer in how well an oil protects against wear because viscosity is not a single number but a function of temperature and load pressure. This gets especially more complicated with multigrade oils, where VIIs can behave unexpectedly, protecting against wear in some cases but causing wear in others. It's beneficial to have better base oils such as PAO or GTL that have higher VIs and PVCs that change less with the temperature. Better base oils such as PAO and GTL also rely less on VIIs to protect against wear, as VIIs can behave unpredictably.