xviper

AP1s have syncros with brass content. GL-5 rated gear lube has high sulfer compounds that can harm these syncros. In the early years, there have been documented cases where GL-5 gear lube was put into S2000 trannies. Those trannies didn't last long.
In the whole history of the S2000, be they daily drivers, garage queens or heavily tracked cars, a heavier oil has not been needed in the transmission. Different needs and different conditions do not mean the need for a heavier fluid in this particular case. The conditions and operational parameters within our transmissions is not equatable to those within our diffs. Search back about 11 years and learn from past experience. However, if you wish to re-write history, please feel free to do so.
This is all I'm saying on this matter. No point in dwelling upon things that have been common knowledge from over a decade ago. It's all there in the archives.

dwight

First, I agree with all the GL-5 comments.

From what I've read 75W90 gear oil is similar to 10W40 motor oil. I'm pumped both with a hand pump and Mobil 75W90LS GL-5 seemed thicker than Honda MTF, but not that too much.

The Subaru BRZ supposedly also uses a Aisin AZ6, and their manual (pg 417) says to use 75W90 API GL-3. So, I'd be more concerned with the additives than the viscosity.

SpitfireS

There is power loss due to the "friction" in the oil itself.
Thicker oil = more internal friction.
There is power loss due to the friction between mechanical parts.
More mechanical contact = more friction.
In the trans, IMO the oil friction is more of an "issue" and not much less mechanical friction will be generated with a thicker oil: IOW you do not need a thicker oil.
In the diff, with the hypoid gears, the mechanical friction is huge (*), so a thicker oil will reduce this, and longer too, compared to diff water.
Diff oil has a tough job to do, there is little of it and it plays a role in the handling of the car.
In my opinion the reduction in oil friction running diff water will be more than compensated - in a bad way - by added mechanical friction.
Don't forget: mechanical friction = metal-to-metal contact = wear.
So a thicker oil will reduce mechanical friction and add a little oil friction but the result is still positive.
(*)The forces can get high enough to make the pinion push the ring gear out the back.
BOOM says the diff.

c32b

SpitfireS : can I take it then to mean I should drain these oils ASAP? I did read some threads where u mentioned 75-140 oils were ok to use depending on your climate. I'm in Singapore where ambient is usually 30 deg C and track temps are closer to 35-40 deg C

rob-2

I run on the track with air temps pushing 110f here in the desert (45c I believe). You're better just to replace the fluids more frequently.

I run stock honda tranny fluid and the recommended weight redline gear oil for the diff.

What has been of more concern for me is the rapid breaking down of low grade motor oil. For this reason I run a 5w40 redline. Keep in mind after the oil exceeds the operating temperature it often thins and the cheap 'fake' syns can actually separate into their base components. Which is likely why you're concerned about tranny/gear oil.

The more you track, the more frequently you should change your oils. I tend to follow the rule of a full on track day takes 5K off the 30K suggested tranny/diff life.

Some of the guys who only track their cars and are involved in racing them will be changing tranny/diff fluids more frequently - say befor every event. One of them around here runs a straight 50weight motor oil. This is probably more than is required for you and Spitfire is going to tell you will damage you motor.

Keep in mind Spitfire and I disagree on the type/use of oils. I am giving you advice gathered from my own personal experience and talking with regular trackers/racers.

SpitfireS

As for the 75W-140 in the transmission: yes.
For reasons xviper mentioned (safe for yellow metals?) and viscosity.
If you think Honda MTF-III or any other MTF is not enough there are (or is at least 1) alternatives, like Redline MT-90.
This oil is still a transmission gear oil and a little thicker.

75W-140 in an OEM diff will work fine, especially in hotter climates.

Now.. rob-2 dragged engine oil into this discussion...
Lets first make the distinction between spirited "hard" driving on public roads, etc, and true track driving.
Because they are NOT the same.
For spirited driving one may refer to the owners manual where there is NO upper ambient temperature limit for the 30 weight.
IOW: why use a 40 weight when you don' t need it?
The cooling system of the S2000 is pretty good, with the oil/water heat exchanger.

If I were to set-up an S2000 for track driving only I would install an oil cooler AND still run a 30 weight.
While keeping an eye on an oil temp gauge and take it from there.
If oil temps stay high, even with an oil cooler, then a 40 weight may be an option.

I've not heard of cheap oil separating, I'm not saying it isnt true, I would like a link to any page describing it.
There was one post a long time ago by a German owner using 10W-60 racing oil during long high rev runs ending up with a scored #4 cil.
Some UOA's of 50 and 60 weights show they - pretty quickly - shear down into 40 weight territory anyway.
But hey.. you have "Racing oil" in your engine.

As for the OCI on track cars: send in a sample and see what the visc @ 100C is after a couple of events.
Base your OCI on this or change to another oil and do the sample again.

For road use: for years people have used dino 10W-30 in their S2000 that sheared down to a 20 weight.
Without engine damage.
Lately the UOA's of 30 weights show they stay in grade a lot better.
Even the "cheap" ones.
Its 2012 you know, aka progress.

c32b

Thanks Spitfire. Food for thought indeed.

Quick question: what sort of temps would be considered "high" for the oils? Anything above 120 deg C?

jetboater

There is no temp too high for the OEM Honda manual transmission fluid--- just check your owner's manual.....

rob-2

My mistake, I recall read 40c track temps and thought OP was running different oils for track day protection.

Drop by a track day and you're welcome you yak up the guys. We've had spun bearings, oil separation, oil starvation but no one is running beyond 50weight motor oil. When failures have happened it's been oil related. Most folks are running spec tranny/diff weights and change more frequently. Sorry I don't have posts. Most of these guys are not coming here for advice so there are no posts.

BTW - I don't buy redline because it's 'race oil' I buy it because its one of a handful of true syn oils. It handles high heat track days well. I don't know of any problem on redline. Similarly, many of the track guys like amsoil. They typically run both motor and diff oils.

If the OP is driving on the streets, stick with stock sir.

SpitfireS

It is not just the temperature, time is also a factor.
Oil running through a crank journal at 9k may reach 150C (HTHS conditions) for a split second.
Oil being sprayed towards the pistons could get hotter I think, but also for a very short time.
That's normal.

I think you want to keep sump oil temps below 120C - engine oil.
As for the diff and trans, I've measured my diff oil right after draining once and found it to be around 70C.
This was after a spirited drive with ambient around 20C (IIRC)
The trans may get a little hotter as there is heat energy from the engine going into it.
IMO if you're not endurance racing diff & trans temps are no issue.

rob-2: the racing oil comment was about the 10W-60 "racing oils", regardless of brand.
Redline is top notch stuff - no doubt.
Their 0W-30 was/is high on my list, especially now the Amsoil SSO is out of production.

IMO, for an occasional track day on street tires, a 30 weight will be fine, I've done that twice, UOA after 15k or so including a track day showed nothing.
On r-comps / semi slicks you need to worrie more about starvation in long sweepers, IMO.
No oil will save you if it is not there.