Yes they were, but not the offset ring and pinion you have in the S2000 diff. I will come back to this further down.

If you want to pass the API GL5 there is a series of tests, they come under a code name (I forget which at the moment) as an industry standard set of tests, pass those and you can claim your oil to be API GL5. I do agree it would be interesting to see what "just" passes and what excels and what is in the middle, but we will never get that kind of depth of info.

So, I decided to look more into GL6 and what I found was quite interesting. After contacting Lubrizol (oil additive supplier) about their standards for API GL6 they dug through their files. The only test records they have for API GL6 was back in 1965 with a Ford Galaxy 427 and they performed 100 drag starts and tested the wear. Then in 1971 they used a Mustang Mach 1 429 and performed another 100 drag starts, then measured the wear. That is it, unlike GL5 that has a whole range of tests it has to pass before it can claim that spec. The other interesting thing it was never specified by any car manufacturer, so despite being in the Honda manual (probably included as the manual writer found the spec somewhere) it is not specified for you diff, it just means if there was one you could use it. Its intended application for was heavy plant, earth movers, quarry stuff that sort of thing.

So why did GL6 not take off if it exceeds API GL5? I wondered this and it turns out it was engineered out, the hard part manufacturers who made diffs etc improved their design to become stronger, more efficient, quieter etc. As a result of this the offset of the ring and pinion gear became much less previous. No need for GL6 it was dropped and the development was as well.

That sounds like more than an oil issue to me!

Not familiar with the LE stuff, having had a quick look the info is somewhat woolly on a few things.

Not necessarily

I think you are right, Honda didn't even look at changing the owners manual, because there wasn't really a need to.

Cheers,

Guy

 

This is very nice insider info, stuff one will never find without knowing someone that knows someone
I guess the 'test equipment that got lost' was that Mustang?
Imagine that manual writer asking "Hey, Tanaka-san, what should we write down to put in the S2000 the diff?"
Tanaka-san: "Something really good Haruto-san, as this is not your typical 140 horsepower RWD convertable"
Tanaka-san was clearly making fun of the 1999 MX-5.
If you read between the lines I reach the conclusion this diff needs good, shear stable oil on the thicker side of the 1999 SAE 90 spec and in 2018 a 75W-110 fits perfectly.

One can say a lot about LE but not that their oils or info are woolly.
They don't deal with automotive people that 'drive their car hard, y'now', they deal with industry where they don't mess around.
If it would be easy to get here I would use it 100%

I think it's also time to reverse the question.
Q-"What is there against using 75W-110?"
You guys even sell it, the Redline 75W-110 is a really good oil, Redline is known to use proper syn base oils (check the MSDS) and their 75W-110 has very good specs (it's 20.9 cSt)
Using thick(er) oil in a diff is not like using a 50 weight 'racing oil' in an engine and the F20 in particular 'for extra protection'.
A simple physics theory mind test will show thicker engine oil lowers the oil flow at higher rpm.
Thicker oil + positive displacement oil pump = higher pressure at equal rpm.
Oil pump bypass opens based on pressure alone, unrelated to rpm.
Combine = thicker oil = less oil flow at higher rpm.

None of this goes for the diff.

 

Well, if GL6 was only in their in the first place simply because some tech writer stumbled across the specs, and threw it in there in a misguided attempt to cover the bases, and we really shouldn't be hunting for GL6 fluids as if they were better for our cars, then perhaps there was a need to revisit the manual, to take out the odd and misleading GL6 spec reference!

 

It always was GL5 or GL6. (Never GL5/6). With the commonality of GL5 there never was any need to be hunting for GL6. GL6 was never better than GL5 in our cars. Meets specifications is just as good as exceeds specifications. I read somewhere that the test platform for GL6 was the Oldsmoble Toronado which was laid to rest in 1992. I'm betting it was the first generation ('66 to '70) with big 425 and later 455 CID V8s and front wheel drive.

-- Chuck

 

Yeah it seems that way. We contacted the other major additive supplier with the same question. They came back with the same info, just bit more detail. Some of it below.

We do have some information on GL-6 API GL-6 denotes lubricants intended for gears designed with a very high pinion offset that require protection from gear scoring in excess of that provided by API GL-5 gear oils. A shift to more modest pinion offsets and the obsolescence of original API GL-6 test equipment and procedures (Ford spec M2C105B shown in table below) have greatly reduced the commercial use of API GL-6 gear lubricants. The key distinguishing test that was different between API GL-5 and GL-6 was the "Ford Drag Start" test that has not been available since the 1970s. The test was conducted in a 3.5 to 1 ratio model C90W-4200AE Ford axle with the conventional (not limited slip) 4-spider differential and 31 spline side gears in a 1971 Mach I Mustang with a 429 cubic inch engine and 4-speed manual transmission with a gross weight of 4780 pounds, with 2456 pounds on the front wheels and 2324 pounds on the rear wheels. The test was conducted with a 50 mile break-in on the new axle assembly followed by 500 full throttle drag starts, going thru the gears with power shifts to a terminal speed of about 106 mph. The test determines the anti-score protection of the axle gear lubricant under conditions of high speed and extreme shock loadings.

Imagine being the poor guy who had to do the 500 drag runs, what a horrible day job

I will add, I need to thank Martyn Mann - Technical Director Millers Oils for helping me get all this information. They even gave details on the additive package required to make a GL6, it can be done still. We just need an old mustang to test it and then pull straws on who will do it

You are spot on with the good shear stable oil, you will get this from any good quality reputable manufacturer. There is still absolutely nothing to say it needs to be the thicker side of SAE90, it just needs to be SAE90 or as you say SAE110 is also suitable because it fell into the original spec viscosity at the time. It is owners preference and nothing else, there is no data from Honda or torsen to suggest otherwise.

As I said I know nothing about them, this came from other oil manufacturers who have come across them in the industrial world. Many oil folk here are always bit sceptical of American lubricants, I would love some to test.

There isn't anything against it, it is a case of either or as they both fell into the original recommended viscosity. The point I have been trying to make is that there is no factual reason to go for the SAE110 in the S2000 diff over SAE90, its a choice that's all and an S2000 diff using quality oil will happily go its whole life on either viscosity. But SAE90 is much more widely available

We have two from Millers as well, they are there because some oil companies see it as a gap in their range if they don't have one. Usually smaller manufacturers to help give them a usp against the big boys and usually put a "racy" spin on it for marketing. Others just haven't bothered because of the lack of market place for it. I will try and find out if SAE110 has actually been recommended for an application by an oem.

I hate that term 50 weight, technically there is no such thing. I guess they mean monograde SAE50, but that doesn't sound as cool. Reminds me of the scene with Johnny Chan from Fast and Furious

Spot on, robs power too. And increases oil pressure, some see this is a good thing but pressure is a measure of resistance. The more you have the more the oil is resisting being pumped around (obviously a balance is needed to keep hydrodynamic lubrication).

Cheers,

Guy

 

Good info Chuck thanks, turns out the test equipment was a Ford Galaxy followed by a Ford Mustang Mach 1.

Cheers,

Guy

 

A couple of comparison pictures from the drain plug magnet.
This is from after one year of use, a few track days and maybe 1000 miles in total. Lucas synthetic 75W-90.


I have also tried the super expensive Honda SAE90 hypoid oil, that one I ran for two years and it came out similar looking to the one above.
This is one year of similar use with a 50/50 mix of the Castrol oils.


Scientific? Not really but it's enough for me to not try any diff water again
I will probably go for a straight 75W-140 on the next change.

 

To flanders: did you notice anything from the rear in handling / behaviour around tighter turns going to the 75W-110mix?
For me the change was back-to-back: notice weird behaviour for a while - go to dealer - change diff oil - drive away - weird behavior gone, instantly.
< me being a happy driver again.
Do those Castrol oils smell the same?
Or does the 75W-140, like the Motul, has a distinct 'rotten egg' sulphur smell and the 75W-90 does not?
The 75W-140 won't let you down

 

Yes, please thank Mr. Mann on behalf of S2ki (credit where credit is due) for the info, very nice to read.
But you said yourself the single grade SAE90's at the time were around the middle and that turned out to be the split viscosity.
The modern multi weight 75W-90 oils are thinner AND have VII, so why take the risk?
IIRC you also mentiond something like this: single grade + vii = one grade up and when it comes to gear oil SAE gave us that smell step with SAE110.

Yeah, I meant the 10W-50/60 'Racing" engine oils.
UOA's on BITOG showed those oils shear down rather quickly into SAE 40 range, btw.

 

@flanders: Are those small metal bits on the drain plug in the first pic or just odd reflections of light?