Viscosity index is said to be an indicator on the quality of the fluid, in how it relates to cold temp and hot temp viscosity range. You don't get the numbers from all manufacturers unless you bug them for it, so I don't use it much for comparision purposes. Most of the high end manufacturers are going to have good viscosity index numbers so it doesn't tell you a whole lot, with only minor variations. High end group 4 and 5 synthetics are going to have good viscosity index numbers.

 

That viscosity chart on BITOG is outdated as it doesn't show the SAE 110 and SAE 190 grades.
(this IS the last time I'm going to mention that )
Its not rocket science to split up the SAE 90 and 140 ranges but, for examlpe, the new highest SAE 90 viscosity isn't shown.
IOW: an SAE 90 in 2009 may not be as thick as a SAE 90 in 1999.
If you can find a single grade, that is
And no: a multigrade is not just a single grade that flows better.
Multigrades have more VII and those VII are not lubricants.
IMO its a good thing to go one step up in grade when going from a singe grade to a multigrade, IOW: can't find SAE 90 -> at least use SAE 75W-110.
Keeping the viscosity range split in mind and the fact that HONDA recommends a single grade and the S2000 diff holds less than a quart of oil..... not a bad thing.
Its your diff.

 

So...Spits -

What are you running in your diff now? I'm looking at the Amsoil 75W-140 next time for my 4.57 Richmonds.

 

In my 4.57 I'm using Amsoil SG SAE 190.
I picked that viscosity as I was preparing the car for a track day, including Carboteck pads and Advan A048 tires.
Note: Richmond Gear are recommending 75W-140 for their gear sets.
I trust their opinion.
There are still 4 quarts in storage so I will use it again.

For an OEM diff, (and IMO) a 75W-110 is "just" a replacement for the OEM SAE 90.
75W-140 would be ok too.

Btw.. in my owners manual, the diff oil recommendation is SAE 90 GL-5 *OR* GL-6.
What does that tell you?
GL-6 was intended to spec gear oils to be used in high offset hypoid gearsets.
GL-6 spec is now not in use.
What is offset: the distance between the centerline of the pinion and the center of the ring.
No offset would mean straigh cut gears, max offset is a worm gear.
(if you know what I mean)
The higher the offset, the more the hypoid gears are sliding, IOW when the pinion turnes its teeth slide over the ring teeth while they push them away making the ring turn.
There is no oil pressure in a diff, like in an engine with journal bearings.
The keeping parts apart all has to be done by the oil film strenght without oil pressure.
The less friction between the pinion and ring, the more the ring wants to turn instead of being pushed out the back.