Originally Posted By: Steve S
I wonder if a 5w-30 really protects much better than a 5w-20 considering the viscosity shear . I would say a straight 30 is tougher than a straight 20 . A straight 30 is tougher than a 10w-30.
You're right about that. Both a SAE 20 and SAE 30 are 'tougher' than even a 10w40 imo.
When talking about multigrades achieved by VIIs and not HVI basestocks, viscosity is not a very meaningful metric at all. VII's make poor lubricants, they shear both temporarily and permanently and they make a mess at high temperatures. Here's an illustration, imagine a styrofoam and water emulsion tests as a SAE 80 grade oil, and yo9u think "oh SAE80, thats heavy duty, nothing will break that film" but as soon as you apply a certain amount of pressure, the styofoam collapses completely and you're just riding on the water, this is exactly the same manner in which VII's dissapoint at high loaded, high shear surfaces. (Not to mention, like VIIs, the burning styrofoam would make a nasty mess.)
A SAE 5w20 oil and a SAE 20 oil are dramatically different from each other, regardless of if they share a common viscosity@100C. "If VIIs are so poor at lubrication, then why arent all passenger cars dying early deaths?" It's simple, passenger car engines can survive on 5wt oil. 5w30, 5w20 are nothing short of 5W oils, beefed up with "space hogging" VIIs. This is why there is virtually no difference in UOAs between 5w20, 5w30 and even 0w10 oils, and more of a difference between additive packs and operating conditions. The fact remains that without the VIIs, theyre all just about SAE 5W or less. Now, multigrades formulated with inherently HVI basestocks (ie. some Gr3, Gr4,5) can attenuate or be exempt from this problem, as there is more/all basestock and less/no VII. This is also why I favour small viscosity spreads in my multigrades and prefer straight to multigrade when possible. Multigrades with wide spreads should come with a warning: "jack of all trades, master of none". SAE20 oil has approx the film strength of SAE20W50 under shear, just without the VIIs. When high performance engines that run 20W50 load up their bearings and cams, and the VII's collapse, it's just a SAE20 film separating parts. Ever hear someone refer to 20w50 as a weak oil? Certainly not. But strip the VIIs from it, and you're left with a straight 20. This is why knowing the basestocks, in my opinion, is very important. Even when the oil is cold, a petroleum SAE20 will protect surfaces much better than a Gr3 5w30. Why you ask? Because the film of the SAE20 can't shear. With advances in technology, that isnt always the case, though. Now, with basestocks becoming more pure and with VIs that can be tailored, less VII's need to be used in the finished product compared to the past. However, if a 5w20 and 5w30 are being blended from the same basestock, you can be sure which one has more space filling VII's, and less basestock but only if you have some insight on the blender. They could theoretically use SAE 10 basestock, allow it to meet 5W multigrade spec with PPDs and extend the VII to SAE30 @100C with less VII's needed than if you began with a SAE 5W basestock. My point is that all oils are not created equal, even if they meet and share their target SAE grade and API/ILSAC performance ratings. One formula will always have protective advantages over the other formula, and it's that 'edge' (no pun) that I personally look for, since operating conditions are never predictable. However, it's not impossible or a lost cause to figure it out, you just need to follow the clues like basestock group, density, reliance on additives, energy conservation approval etc.
