Is 75w-90 too thin for the differential?
 

I seem to be getting a bit of the commonly reported differential whine under acceleration, you can't hear it with the radio on, but I'm going to try the LE 1605 from hardtopguy. Will that fix it?

Gear whine will increase as the temperatures get warmer. LE1605 will help, but isn't much higher in viscosity than an SAE90. You will still get some whine, but not as much. I'm considering trying a SAE140 this summer in Texas.

SAE 75w90 is the same weight as SAE 90 once warm which is always in NC.

i use 75w90 mobil 1 syn

Amsoil SG 75W110.
Not too thin, not too thick - it's the 'Goldilocks' of gear oils with serious synthetic credentials.

Haha, Goldilocks.

I call BS.

SEA110 is the old SAE90. So what you buy today as 110 passed for 90 just a year ago.

SAE 90 is a range.
As per SAE J306 gear oil spec.
http://home.tiscali.nl/onlinestorage/SAE%20J306.JPG
This is the june 2005 (revised) spec.
Before the SAE 90 range was from 13.5 to 24 cSt.
That range was to wide and they spliced the SAE 110 in there.

Note: the S2000 was designed in 1998, with the old SAE 90 range in mind (IMO).
And what does Honda recommend for the 7" diff that only hold 900cc's of oil with a 240 HP engine in front of it?
A single grade SAE 90.
And that could be up to 24 cSt.
LE607 is/was 21 cSt.

Anyway....
75W-90 means the oil is within that spec.
Nothing more.
It must be between 13.5 and 18.5 cSt at 100C.
Most datasheets I have seen of recent multigrade gear oils showed they are around 14-15 cSt.

First place you look for oil specs is the manual.
Personal experience gathered by experienced mechanics is another.
There are a lot (and I mean A LOT!.. could be millions now :p ) of cars that have no problems at all with Mobil1 75W-90.
But its not the same as SAE 90.
That is a fact.
Period.

Its your car, you decide.

SAE 110 falls in the upper range of the old "90"... that's why it's perfect. There were 90s that fell within that range and 90s that did not. 110 is specifically right in the sweet spot this car requires - hence my earlier point.

I use Amsoil 75W110.

75w-110 is probably a good pick, I tend to side with slip on this one. I suppose it depends on how hard you drive the car, for many people, 75w-90 is going to be good enough (a little gear whine perhaps) while offering slightly improved mileage.

It's amazing how many old threads there are on this issue.