any idea why honda didnt think about that? I have never read that any diff that failed was obviously correlated to 90 grade oil. Most US drivers seem to use 75w90.

 

There were probably fewer manufacturers selling synthetic 75W-140 in the late 1990s, when the S2000 was developed.

As for mileage, I put a 4.11:1 in one of my old BMW 2002s (OEM is 3.64:1). The one with the 3.64:1 got around 20-25 MPG or more. No matter the driving type, I could never get over 22 MPG with the 4.11:1. Both transmissions had the same gear setup.

 

Thnz for your input. As far as I know Honda is still in business. If they would think that differentials are breaking due to the wrong oil, dont you think they would send out a reccomended update? As far as I know the 5w40 option for the engine is an official addition by Honda. Unless I got that wrong? Which suggests they do updates when they think its neccesary.

why wouldnt they for the diff if its true that a 110 or 140 is much better and dealers are still pouring in the standard 90?

I am not familiar with bmw and it seems you talk about 2 different cars? Did you do mainly city or highway driving with the updated diff? Also I dont know if the bmw has a tal 6th gear or if its running in high rpm on highway like the s2000 does?

 

You have to keep in mind what the world was like in 2000. They suggested 5w40 as an option as it was likely to be full synthetic , and in theory it would flow better in winter cold temps compared to a conventional 10w30 oil, so they weren't so concerned with the 40 part of the spec.

On the diff they spec'd an SAE 90 fluid, that is actually a pretty thick fluid going back to year 2000, comparable to an 110 fluid in today's viscosity charts. They would not have recommended a 140 weight as it was overkill and and not very common back then, even today few manufacturers would spec a 140 fluid. Thicker is better when it comes to the diff in terms of wear rates, but not in terms of breaking, diffs with 110 fluid would not break due only to fluid choice. Diffs break due to abuse only, and fluid won't save you when they are abused, they rarely die from normal use, so a 140 fluid isn't going to save a diff if abused. Saying that you are safe using a 110 fluid or 140 fluid, totally up to you, you can't go wrong with either. 140 may have slightly better wear rates, and don't count on Honda ever caring about what alternative diff fluids people "could" use, they don't care, and they won't go back to change their recommendations. They won't recertify cars in order to change recommended specs unless it was a dire situation.

 

Yes, 2 different BMW 2002s. 4 speed transmissions. "No matter the driving type (or style of driving), I could never get over 22 MPG with the 4.11:1. "

 

it sounds unreal no matter the driving type? If the FD is higher, the acceleration is easier on the engine and should be more efficient in stop and go. Less on high speed. In theory dont you think? Do you have a theory as why it was always less effixient in mpg?

 

thanks for the info. From changing the fluid in the diff we can conclude there is no wear. Maybe microns. Like the engine but probably less. So than I wonder, what is the downside of a 75w90 ? There is theoretically less powrloss and better effixiency. Also the torsen works better in thinner oil. Although Torsen says we probably wont notice and I dont even know what the function of the Torsen is. But it works less with thicker oil they say.

I read corvettes also use 75w90 in the diff. Is there something special about our diff that needs "more protection"?

 

No downsides for the street

Torsen purpose is limited slip. More even power distribution to both rear wheels.

Without limited slip, power goes to only one wheel at a time.

There are other limited slip technologies. Torsen is especially nice bc no wear parts, clutches, etc. All down with clever gears. Downside is its stops working if one wheel off ground.

So like awful for offroad, where that is common, but also not great on track when one rear wheel unweighted in corner.

Only a thing w super sticky tires capable of putting car on two wheels.

 

The rear-end on my S runs hotter than any car I've owned in the last 55 years. I have ran 75/140 Mobil 1 since the day I bought it 15 years ago because of how hot it gets. Have put 200k miles on it and my drain plug is always clean.

Rod

 

I read on another forum that somone mentioned that 140 will run hotter than 90 because of the thickness. So in your case can I ask, did you temp it with 140 or did you temp it hot, then started using 140 and then saw temps going down?

if the viscocity of 140 is indeed really thick I could believe the extra stixkieness can cause more drag and heat.

On truck forums they say 140 is only advised for heavy hauling applications. So perhaps high speed is not what its made for. Altough many people here seem to use it.

Im very curious about more details in your situation!

ps. I ran it aboit 8-10 years without ever changing it and It was filled before me with I guess dealer stuff. And when I changed it myself, bolt was clean as well... You mean clean from metal parts right? (I dont track just some spirited driving and occasional donut)