OK - I spoke to the dealer again - they said it was not Castrol magnatec as indicated on the invoice, but 'Dual Pump Fluid', made by Honda. I asked if this is the same as they use in other vehicles, for example the CR-V and, worryingly, they said yes. However, they assure me this is the correct fluid, so I don't see there's much I can do to change their mind. On their head be it!

Thanks for all the advice.

 

Their dual pump fluid is the WRONG fluid. They should have an "in-house" memo stating this. This memo is from Woodwork, who is a board member here. Woodwork also happens to be Honda America's Technical Division leader in charge of the NSX and S2000 exclusively in North America.
They will soon be putting in a free rear diff for you as the damage has already been done. Woodwork has even posted in these forums specifically stating to NOT use this fluid.
Here are some threads you should see:
https://www.s2ki.com/forums/index.php?showtopic=115704
http://forums.s2ki.com/forums/index.php?showtopic=199536

Since I'm in Canada, I don't have the American memo, but here is the Canadian one:

 

What a fiasco! Yes, I suspected this might be the wrong fluid, based on the excellent advice in this forum. The difficult job, though, is in convincing my dealer. I have read the threads, searched google, and have printed out the memo you posted, but I can't actually find anywhere the specification of this 'Dual Pump Fluid'. Nor can I find any 'specific' evidence that it is the wrong fluid for the s2000 diff. I agree that it is implied by the memo, in that it says that the fluid recommended for the S2000 is not suitable for the CR-V, but it doesn't specifically state the converse. It's enough to convince me, but I really don't think it's going to be enough to convince a UK dealer, especially as it's not a UK memo, and also as they seem to be confident that their fluid is suitable.

I could of course just get the fluid changed by a third party, but then I would have less comeback if things go wrong.

So, the only thing I can think of left to do, is to contact Honda UK customer services tomorrow, ask them what they consider to be the required fluid, and ask them for the specification of fluid that's been used. Theoretically, they should be willing to cooperate, as it is in their best interest to ensure their cars work properly. However, unless they can 'overrule' my dealer, I think I'll just have to let them replace it if it breaks (again!)

 

See if you can get Honda UK to contact Honda USA technical division in California. That will set them straight for sure.

I'm sure you've seen this but just in case, print this out and show it to them. Tell them to contact Woodwork directly.

VTM-4 is designed specifically for the Acura MDX and the Honda Pilot (electronic limited slip) rear diff.

It is not GL5 or GL6 rated.

Do not use it in your S2000.

HTH,
Woodwork

 

VTM-4 .... that's not the same as 'dual pump fluid' though is it? Sorry - I'm not technical so I'm not sure what VTM-4 is, all I know is that 'dual pump fluid' is what's gone in this time.

 

Dual pump fluid = vtm4* = you're going to need a new diff sooner rather than later. Sorry.

*Honda VTM-4 (Variable Torque Management 4WD) full-time four-wheel drive with electronically locking rear differential. It uses "dual-pump fluid."

 

Right - thanks. If they do agree to flush that out and refill it with a GL-5 oil, will the diff be OK? XViper - you said the damage will already have been done ... but I will only have done 50-60 miles with dual pump fluid in it.

 

I'm not saying that this will happen to you, but here is a post from "Incubus" from that first link I put up:

Hey, my dealer put that stuff in my differential and...

Now I've got a new one.

Less than 60 miles after the change, I started hearing the most horrible whining sound.

Do not put that stuff in your car.

 

I have been using Mobil1 75W-90 synthetic gear oil since the beginning. I drive the car quite hard (clutch drops while moving/turning on a regular basis to kick out the rear). Metal filings on the drain magnet have been non-exsistent, and the fluid drains out as clean as it went in after 10k miles. So this has proven to be a great choice and is available all over. Im now running LE607, based on working with it and Road Rage's extensive research. It appears to be the ultimate in protection and reducing driveline shock. I'd bring a bottle of either one to the dealer when they give you your next free dif

 

I have been running with VTM-4 in my diff for over a year with no problem. It has survived at least a dozen autocross weekends and close to 10,000 miles while I wait patiently for it to self-destruct, drunk on VTM-4.

See this thread