Yes, 1999 is a long time ago (we're getting old) but that's the time Honda wrote the S2000 owners manual stating to use SAE 90 GL-5/6 for the diff.
Holy Grail question
There will be no problem anymore once you start using SAE110 in the S2000 diff
It's linked though.
Just look up the 100C viscosity of these 3 oils, Mobil1 75W-90, Amsoil 75W-110 and LE-1605.
(never mind, I will tell you)
Mobil1 75W-90 = 14.6cSt
Amsoil 75W-110= 20.9 cSt
LE 1605 = 20.8 cSt.
Only LE lists the AGMA grade of their oils.
And guess what, the thicker the higher the EP rating.
Out of the 3, what would you pick for the S2000 diff?
And remember: the original Honda recomendation is a single grade SAE 90, GL-5/6
In my opinion, the translation of this is NOT a multi grade 75W-90 but at least 75W-110, for the same reasons you mentioned, addition of viscosity index improvers, etc, etc.
So I refer to any below 18 cSt multi grade gear oil - to be used in the S2000 diff - as diff water (it's partially fun, I hope you get this)
The only thing I will say about engine oil for the F2x's here: get a full true 10cSt syn and get a 0W to get max flow when cold.
(but one has to look at the numbers because nowadays I'm 'sure' 0W oils are marketed/labelled as 5W as the general public still thinks 0W is....waaaaay to thin)

 

SAE specs are cumulative. Spec XX-2 meets all the specs for XX-1, etc. GL6 meets the GL5 spec and GL6 is only obsolete due to lack of test equipment. Since the test equipment is either rare or non-existent I'd not use it even if I could find it. Same for motor oil. API SN meets the lubrication specifications of all preceding specs going all the way back to SA which isn't suitable for engines built after 1930 (darn near 90 years ago).

I've not seen a current chart for the no-longer-GL5 LE1605. The former oil formula was SAE-110W. 100°C viscosity of LE-9920, a certified 75W-140 GL5 oil is 24.8 if you want to run that "thick." A shame Honda didn't list the cST spec for the differential (or anywhere else).

-- Chuck

 

I have no doubt the 'new' LE1605 will also be an ISO 220 oil.
And I've never ever seen any automotive manual specifying cSt range for oil.
Where would one buy an 18- 20 cSt gear oil for example, based on the label on the bottle?
We're stuck to the SAE range.
And that's ok, for gear oil SAE cut the 90 grade range in half to be more specific.
Too bad that happened after Honda wrote the S2000 manual.
Also too bad Honda never updated it.
That said: if they would have we would not be having this discussion and I would actually be working...

 

The new LE-1605 is indeed ISO-220 which is the same as SAE gear lube 90 (LE says it's SAE 110) so it meets the "90" requirement but not the GL5 requirement. The new LE-1605 is specifically not GL-5.

This is the new bottle:


The ISO viscosity measure is much tighter than the SAE. See the data linked below:

https://www.glennbennettcorp.com/hub...-Reference.pdf

Note how SAE 90 covers a very wide band of ISO grades ranging from ISO 150 to 320. SAE 110 doesn't show on the chart but is somewhere between 90 and 140. The chart shows the general cST measures making it especially helpful.

-- Chuck

 

Honda never updated it because there is no need to do so. When the manual was written the viscosity for an SAE90 ranged from 13.5 to 24.0 cst at 100c, so this by default means Honda were happy for an SAE90 anywhere in the viscosity bracket to be used, and most sat around the middle. After the viscosity change there is still 13.5 to 18.5cst for an SAE90, still well within what was originally recommended. With the API GL5 and GL6 recommendation that was just saying it is happy on either or, and as GL5 is still going, no need to update anything as it still falls within the original recommendation. Having had a quick search of the net, it is not full of discussions about S2000 diff failures and the ones I have seen cannot be attributed to using SAE90 over SAE110, so I go back to what I have been saying. Use a decent quality SAE90 (or 75w-90) or 75w-110 if that is your flavour and your lsd will be a happy bunny. Either or it makes no real world difference to this diff.

If you fancy GL6 performance give the Fuchs Titan Race LS90 a go

Cheers,

Guy

 

 

i know mag hytec buys a ton from them, and i i wonder why LE told MAG HYTEC that the LE1605 did not change formulas and that it went out of GL5 spec due to specs changing and that it will change again.....

 

I guess that depends on the definition of 'happy'. I mean, did they really have much choice in their spec? If for instance they felt it ideally ought to be on the mid to higher end of the 90w spec of the day, how could they have spec-ed that? I don't think there would have been a reasonable way to.

Perhaps Honda could confidently spec 90w, knowing that any choice, with any viscosity in the allowed 90w range, would not blow up any diffs (hence happy), even if the ideal fluid, that would have the least wear and least other negative effects, were at the higher end of the 90w spec.

That of course is pure speculation. I'm just pointing out that just because they spec-ed 90w, does not automatically mean anything that met 90w spec is automatically just as good for our diffs as anything else within that original 90w range.

I hope we're all on the same page that the discussion isn't if this fluid or that will blow up your diff (because hopefully everyone agrees that it won't), but rather what would the ideal fluid be, and why.

I know I'm learning a lot from this thread, and I want to thank everyone that is contributing to it!

 

Has anyone ever tried to contact Honda USA or Honda Japan for support and ask this question to them? Maybe there is some kind of tech support. I havent checked yet. To get a final answer, and end all speculation ;-)

 

Ask who at Honda what question? What's "best?"

Could also ask what's the "best" gasoline or motor oil or gearbox oil. But then they'd know it was a prank call. Of these only Honda MTF is mandated (and commonly ignored by those here). Gasoline, motor oil, and differential oil are specified by spec, not brand, that's the reasons the spec exists. We know the SAE Gear Spec 90 used to be wider than it is presently. No reason to not stick with the original spec which was extremely wide.

Now if you could get Honda to commit that it ain't necessary to use a GL5 or GL6 spec oil...

-- Chuck