What's with the super-high VI gear oils?

[dparm]

I'm noticing that there are gear oils out there with extremely high viscosity indexes. There are at least four with a VI over 200 and plenty more above 180.

How is it that a 75w90 gear oil (roughly equivalent to a 10w40 motor oil) can have such a high VI? There are other 75w90s that are below 150, though I'd say the average is around 175 or so.


A few worth noting:

Motul Gear 300 -- 222
Gulf Syngear EP -- 220
Castrol TAF-X -- 210
Idemitsu Racing MTF/Gear Oil -- 210
Maxima Syn Gear Oil -- 199
Shell Spirax HD -- 195
Torco SGO -- 195
Shell Helix Racing Gear Oil -- 194
Eneos MT -- 188
RLI Bio-Syn LS -- 187
Lubro-Moly Vollsynthetisches Getriebeol -- 185
Red Line MT-90 -- 185

 

[Rand]

90wt is a really wide range.
it goes from barely over 30wt to above 50wt.

a large vi is a good thing as long as it doesnt shear excessively.

some Modern ATF's are usually around 200 also.

 

[dparm]

I understand the benefits of high VI, but I'm asking why it is that gear oils can so easily have a VI above 190 and over 220 in some cases.

 

[dailydriver]

Red Line's 75W-140 is also a 188 VI (one of the reasons I mixed it with their 75W-90 NS {176 VI}, and am using it now).

I guess CATERHAM's influence even got to me regarding gear oils as well.

I may eventually try the Motul as well.

(I'm hoping that they BOTH have their high VIs without adding too much (if any) VI improver, due to their high ester basestock content, something you DO NOT WANT, no matter HOW good the VIIs, in a VERY HIGH SHEAR environment like hypoid gears and various hot running l.s. diffs.)

 

[dparm]

Motul Gear 300's datasheet specifically addresses that:


100% synthetic extreme pressure lubricant for an efficient anti wear protection, a better resistance at high temperature and a longer life time.

0% shear loss : Unshearable oil film in extreme conditions. Stays in 90 grade after KRL 20 hours shear test as requested by SAE J306 Standard, July 1998 update

 

[dailydriver]

Originally Posted By: dparm
Motul Gear 300's datasheet specifically addresses that:


100% synthetic extreme pressure lubricant for an efficient anti wear protection, a better resistance at high temperature and a longer life time.

0% shear loss : Unshearable oil film in extreme conditions. Stays in 90 grade after KRL 20 hours shear test as requested by SAE J306 Standard, July 1998 update



Yes, I know, and read that same spec sheet.
I actually might try their 75W-140, or even their made for Nismo Racing 75W-140 (higher yet cSt @100*C) stuff (even though it must be bought at a Nissan dealer for a ridiculously exorbitant price) in the summer.

 

[dparm]

I ought to send in some of my Gear 300 (both in the trans and rear diff) to see how they've held up. Both have about 10,000 miles of hard street use and some track days + autocross.

 

[tdi-rick]

Gear 300 works very well in the one gearbox I use it in, it definitely outlasts the very good Castrol Syntrans 75W-85 I previously used which appeared to be shearing by around 18,000km.

 

[Coprolite]

dparm- Since you found all the VI values would you happen to know which of them also has the lowest viscosity to help figure out the "lightest" in operational conditions?

My car calls for API GL-5 Synthetic Gear Oil Viscosity SAE 75W-90. It has a VLSD, so I don't need to consider using any LS additives. I don't mean to pull this thread off topic, but viscosity measurements would help in the decision process along with shear resistance.

 

[Coprolite]

I did a quick spec look up of the commonly available oils and it looks like the Motul happens to be the lightest along with having the highest VI.

 

[SubLGT]

Originally Posted By: dparm
I understand the benefits of high VI, but I'm asking why it is that gear oils can so easily have a VI above 190 and over 220 in some cases.


Paging Molakule for an answer.

 

[JasonBraswell]

Mobil 1 75w-90 is only 142 VI.
Does this mean it is of lower quality then the OP's list of gear oils?

What should be the min VI for a good synthetic gear oil?

 

[Capa]

I wonder if the value of viscocity index is as important in gear oils as in motor oil?

 

[MolaKule]

Quote:
but I'm asking why it is that gear oils can so easily have a VI above 190 and over 220 in some cases.


The viscosity index is high due to three reasons:

1. The use of a thick (high viscosity) base in the base oil mixture. Higher viscosity base oils generally have higher VI's. The base oil mix is composed of two or more base oil viscosities.
2. More gear lube formulations are using more higher group base oils such as Group II and II+, PAO's, esters.
3. Polymers are being used for friction reduction that have high viscosity indexes.
4. Visosity Index improvers that are being used are better and more stable, and have higher viscosity indices.

Check out this white Paper:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=729289#Post729289

Gear lube formulations haven't changed all that much since this was written. Most advances have been in the addition of new polymers, additive packages, and higher viscosity base oils in the mix.

 

[dailydriver]

Originally Posted By: Capa
I wonder if the value of viscocity index is as important in gear oils as in motor oil?


EXACTLY what I asked in my thread on this topic!!

MolaKule??

 

[MolaKule]

In my view it is since it's a closed system without much cooling, save for conduction and some convection (slip stream) cooling.

A high VI means better cold weather lubrication and less chance of channelling when cold, and more importantly, less thinning when hot.