Viscosity Index: The double edged sword?

[RamFan]

A higher VI is a good thing because it means there is less change in the oil's viscosity at different temperatures. However a high VI may mean a large amount of VII used. This is where my questions begin.

Will a high VI oil that relies on VII shear quicker out of grade than an oil with a lower VI? (My assumption is no)
Is there a difference between synthetics and conventionals in this respect since group III basestocks have a higher starting VI than groups I & II? (My assumption is yes)
Is it possible that while looking at conventional oil it may be more beneficial to look for a lower VI since that may indicate less VII used? (My assumption is no)
Meanwhile, while looking for a synthetic higher VI would be better as this may indicate a higher quality basestock? (My assumption is yes)

If anyone could offer some clarification it would be greatly appreciated!

 

[chubbs1]

It is sometimes difficult to assess the amount of VII's in any given oil. PAO and Esters will yield the highest natural VI. Oils with a lot of VII's will tend to shear more quickly compared to a lube with high natural VI under similar operating conditions.

For conventional oils I have thought of the same thing you mentioned, but in the end I just choose an oil that is made by a Major and call it a day since they will all be somewhat similar.

One good example (at least my guess) of an oil with a higher natural VI may be Mobil 1 Extended Performance which actually has a lower VI than the "regular" M1. I assume the EP has a more natural VI.

The bottom line is use your good judgement and use a top brand oil that has proven itself (fill in the blank) in use through UOA etc.

 

[RamFan]

While we can agree that a higher VI is a good thing as far as viscosity stability. I wonder how short lived that stability is and how much good looking at VI even is when comparing oils. We like high VI but that could just be an indicator of an oil loaded with VII. We dislike low VI but that may be an indicator of reliance of primarily basestock VI with fewer VII added which should withstand the heat and stress for a longer period of time. Very interesting topic imo.

 

[Jim Allen]

Bear in mind that VIIs are better now than they ever have been, so a multigrade oil of today with a high VI is not nearly as shear prone as those of years gone by. That's not universally true, of course, but generally true.

 

[RamFan]

Originally Posted By: Jim Allen
Bear in mind that VIIs are better now than they ever have been, so a multigrade oil of today with a high VI is not nearly as shear prone as those of years gone by. That's not universally true, of course, but generally true.


I'm glad you brought that up. I meant to include that in my OP but it slipped my mind at the time. We definitely need to be aware that there is a quality element to VIIs, there are both low and high quality VIIs that can be used to boost an oil's VI. This further complicates matters since we, the consumers, will never know which was used.

 

[Mitch Alsup]

The other consideration is that you can pick oils towards the thick (heavier) end of things, and then they will shear back towards the lighter end of the same scale while remaining "in grade".

But to do this requires some research as none of this info is found on the can/bottle.

 

[dailydriver]

Paging; CATERHAM. (Suprised you're not already on here!)

 

[RamFan]

Originally Posted By: dailydriver
Paging; CATERHAM. (Suprised you're not already on here!)


I thought he'd be one of the first people to reply!

 

[CATERHAM]

Originally Posted By: RamFan
Originally Posted By: Jim Allen
Bear in mind that VIIs are better now than they ever have been, so a multigrade oil of today with a high VI is not nearly as shear prone as those of years gone by. That's not universally true, of course, but generally true.


I'm glad you brought that up. I meant to include that in my OP but it slipped my mind at the time. We definitely need to be aware that there is a quality element to VIIs, there are both low and high quality VIIs that can be used to boost an oil's VI. This further complicates matters since we, the consumers, will never know which was used.

That's pretty much what it boils down to, the quality of the polymer viscosity modifiers that are used.

That said the new crop of 200+ VI oils from Sustina, SynGard and Toyota are only possible due to very recent advancements in polymer technology that are polymethacrylates (PMA) based. Lubrizol introduced their very high VI star-like polymer PMA called "Asteric" in 2008. Germany based Evonik has also patented a very high VI PMA with a comb like branched structure that are used in some Japanese engine oils and ATF. These PMA based polymers are of low molecular weight and work most effectively with light base oils to formulate very high VI, shear stable, 0W-20 and light 0W/5w30 oils.

Most polymer thickeners used today are olefin copolymers (OCP) that offer good performance at a reasonable cost to the blender.
Shell uses styrene-isoprene polymers they call "Shellvis" that have some advantages in shear stability and low-temp' performance.

Most muti-grade oils used today are quite shear stable particularly in the lower grades. Probably the most shear prone grade is the GP III based 5W-50 since most examples use high molecular weight polymers that tend to be the most shear prone.

Regarding economically priced conventional oils, one would think that the low VI 10w30 grade would be more shear stable than the higher VI 5w30 grade, but in practice that is not always the case. In fact the 10w30 oil may use just as much if not more VIIs in it's formulation. 5w30 oils tend to use higher quality base oils and higher quality VMs. Additionally, the 10w30 grade is usually just a heavier oil with most formulators, which is why in part the grade may have a higher NOACK and higher HTHSV.

Choosing a lower VI oil on the assumption that it should prove more shear stable in service is really a counter-productive practice with the oil choices available today.

 

[RamFan]

Thanks for the post! I learned a lot, however for the most part 10w30 oils have a lower NOACK than 5w30. So that makes me wonder if its because they rely less on VM. This can be seen in the comparison chart that I made; https://bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2735022