Gearbox Oil

**Warb** · 4th November 2015, 08:21 AM

From what I can establish, the "W" rating on gear oils (which supposedly defines the low temperature "winter" viscosity) is actually based on the temperature at which a defined viscosity occurs. As such it will also impact the curve of viscosity against temperature, but because the changes are very rapid at the low temperature end of the graph, the real-world impact of going from 85W to 80W is probably fairly minimal other than at temperatures towards freezing point and below.

In reality, the rate of change in viscosity is very rapid at low temperatures, and then starts to stabilise. So an 80W/90 gear oil may be 150,000cP at -26C, 150cP at 40C and 15cP at 100C. So apart from a gearbox being driven from cold in low temperatures, the "W" figure is largely meaningless. However because of the difficulty of making mineral oil behave this way (and remain stable under shear), most gear oils below 80W/x are synthetic or semi synthetic.

To me, what seems important is the 40C and 100C viscosity, as these figures are more relevant to my usage. I am far more likely to drive a Series LR in the summer, when the ambient temperature is >25C.

40C and 100C viscosities are below:

Castrol EP90 GL4 - 178 / 16.4 (not available in Australia)
Penrite 80W/90 GL5 - 152 / 15
Penrite Mild EP GL4 - 320 /24.2
Redline MT90 75W/90 GL4 - 90 / 15.6
Nulon 80W/90 GL5 - 137 / 14.5

As expected, the multigrade oils are all thinner at high temperatures than straight EP90. The Widman document suggests this is because they are not true multigrades with viscosity modifiers, but are simply blended towards the lower end of the acceptable high temperature range, to ensure they also meet the low temperature requirements. It is also interesting that the Redline MT90, whilst thinner at low temperatures is actually the closest to EP90 at 100C. The viscosity specification for SAE90 gear oil is 13.5-18.5 at 100C, SAE85 is 11-13.5, so the Nulon product is actually at the low end of SAE90.

Unless the viscosity/temperature curves differ radically, it seems likely that at any given operating temperature the viscosities of the non-synthetic oils will remain in the same relative order, so the Nulon 80w/90 will always be the thinnest, and all the multigrades will be thinner than EP90.

Penrite Mild EP, at 24.2cSt, is actually just inside SAE140 (24 to 32.5), even though they classify it as SAE110, but both Penrite and Roamerdrive recommend it. The reality is that this just means it has the same viscosity as the other oils do when they're a few degrees cooler!

Personally I'd prefer to avoid any possible GL5/yellow metal issues, so I'm going GL4. Whilst Mild EP is, on paper, outside "EP90" specification, to me the downside of a slightly stiffer gear change for a few minutes on a cold morning is outweighed by the reduced noise (and Roamerdrive recommendation) the rest of the time. Given the time, effort and expense of restoring a classic car, I'm willing to splash out on the ha'p'orth of tar....

**gromit** · 5th November 2015, 06:29 PM

Not sure about the tolerance on the SAE categories but a few years back I was after a straight 40 and the Shell technical helpline suggested their agricultural 30 (for lawnmowers) was at the top end of 30 and very close to straight 40.
That would explain why different manufacturers products have different viscosity but still call it 'SAE 90'

Some info here Motor Oil Viscosity Grades Explained in Layman's Terms

For years I've used straight EP90 but now there is 75W90, 80W90 etc. etc. which supposedly offer better gear selection when cold.

I buy direct from a blender (much, much cheaper than SuperCheap etc.) and they felt that their GL5 was 'OK' for older gearboxes but then offered to blend some GL4. This suggests that how they get the GL5 spec. may differ from company to company but they weren't willing to take the gamble.
It's been GL4 for me ever since.

Here's the 'Widman' article mentioned by Warb http://r.search.yahoo.com/_ylt=AwrSb...CfH1nMhykcsdM-

Colin

**whitehillbilly64** · 5th November 2015, 07:08 PM

Been looking at this also,

Castrol Manual Vmx 80w | Manual Transmission Fluids | Car | Products | Home

whitehillbilly

**gromit** · 5th November 2015, 07:31 PM

Confusing though.......

"SAE 80W viscosity ? SAE 75W-80 extrapolated (equivalent to 10W-30)" ??

Colin

**mick88** · 5th November 2015, 08:10 PM

extrapolate
ɪkˈstrapəleɪt,ɛk-/
verb
past tense: extrapolated; past participle: extrapolated
extend the application of (a method or conclusion) to an unknown situation by assuming that existing trends will continue or similar methods will be applicable.
"the results cannot be extrapolated to other patient groups"

estimate or conclude (something) by extrapolating.
"the figures were extrapolated from past trends"
Mathematics
extend (a graph, curve, or range of values) by inferring unknown values from trends in the known data.
"the low-temperature results can be extrapolated to room temperature"

**Warb** · 5th November 2015, 08:14 PM

Originally Posted by gromit

Confusing though.......

"SAE 80W viscosity ? SAE 75W-80 extrapolated (equivalent to 10W-30)" ??

Colin

Gearbox oil viscosity ratings aren't the same as engine oil, so presumably SAE75W/90 gearbox oil has the same viscosity as 10W/30 engine oil. And the "multigrade" claim of 75W/80 simply means it meets both the 100C requirement for "straight 80" and thickens to 150,000 cps at the right temperature to meet the 75W classification.

It is entirely possible that an EP90 could also be classed as xxW/90 if it stayed "runny enough" at low temperatures. In the past, people only worried about how thin the oil got at high temperatures, but now they specify the low temperature end as well. Unfortunately for those of us driving old technology high mileage vehicles, the oil manufacturers chasing the W rating means the oils are often at the low end of the SAE 100C standard.

**mick88** · 5th November 2015, 08:17 PM

Originally Posted by gromit

Confusing though.......

"SAE 80W viscosity ? SAE 75W-80 extrapolated (equivalent to 10W-30)" ??

Colin

Very, very confusing!
But remember, back in the 1970's Caltex petrol (CX3) had liquid tungsten in it!

Cheers, Mick.

**gromit** · 5th November 2015, 08:37 PM

Originally Posted by Warb

Gearbox oil viscosity ratings aren't the same as engine oil, so presumably SAE75W/90 gearbox oil has the same viscosity as 10W/30 engine oil. And the "multigrade" claim of 75W/80 simply means it meets both the 100C requirement for "straight 80" and thickens to 150,000 cps at the right temperature to meet the 75W classification.

Doesn't it thin to the 75W ?

As I understand it that the 75W is the 'winter' or cold viscosity rating and through additives it thickens to an SAE 80 at 100C.

Colin

**whitehillbilly64** · 5th November 2015, 08:46 PM

I thought the same Gromit.
Lighter weight cold/startup, allowing the oil to get around easily, then thickening when warm
So would sae30 motor oil, do the same as EP90 transmission oil ????
Getting confused now..

Whitehillbilly

**gromit** · 5th November 2015, 08:48 PM

Viscosity explained.

Oil Viscosity Explained

SAE90 gear oil is the same viscosity as high SAE40, SAE50 or SAE60 motor oils.
SAE75W Gear Oil is the same viscosity as SAE10, SAE15 or up to mid SAE20 motor oils.

Turns out rather than thickening, the rate of thinning is slowed down in a multigrade. So 75W/90 cold acts like a 75W but doesn't thin to the viscosity of a 75 when hot but to the viscosity of a 90 instead ?

I'll just keep putting the correct grade of oil in, it's worked for me so far........

Colin

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