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Ok, got a new to me NC-2. 106,000 km on it, I'm baselining all the fluids. Engine oil, transmission, diff, coolant, Brake/clutch fluid. The manual and the cap on the shared brake clutch reservoir says DOT3. This car is going to see the occasional 15 minute sprint session, maybe 5 or 6 track days a year. Should I believe the Mazda engineers and put DOT3 in it or go for for a good high temp super DOT-4. I mean I run Dot-4 in my NA-8! Why would Mazda specify DOT-3?

Your NA8 is also specified for DOT3.

Put DOT4 in.
Or better yet SUPERDOT4.


And as to why Mazda Specifies DOT3.
The answer is cost which may sound stupid at first but this is what I remember a Japanese Mazda engineer quoting about servicing in the past.
Mazda basically specifies a grade of fluid (be it engine, transmission, oil etc etc) that meets a cost criteria while meeting a minimum technical standard.
It is not necessarily the best oil or fluid to use.
You need to keep in mind that when this is specified it is based on full RRP prices.
That is why a lot of their cars did not even specify Synthetic motor oil over the last few decades.

Interesting, I suppose over a million cars they might be able to save a million dollars. Super DOT-4 it is then.

I think most cars sold for general road use specify Dot 3, because they don't need anything higher. Good idea to use Dot 4 for track use though.

So NC's have a shared brake & clutch reservoir. I presume the fluid is separate & not the one fluid? The reason I ask is because an RS275 Megane was having trouble with his clutch last weekend at Challenge Bathurst. The driver said it was because of the hot brake fluid which also operates the clutch.

RS2000 wrote:I think most cars sold for general road use specify Dot 3, because they don't need anything higher. Good idea to use Dot 4 for track use though.

So NC's have a shared brake & clutch reservoir. I presume the fluid is separate & not the one fluid? The reason I ask is because an RS275 Megane was having trouble with his clutch last weekend at Challenge Bathurst. The driver said it was because of the hot brake fluid which also operates the clutch.

They are combined on the NC.
Never had a problem with it.

Piss easy to bleed the NC clutch fluid as it is the same the same sort of fitting as the calipers.
Well easy for me as I cheat with my home made vacuum pump setup.

The real question is did the Megane driver have his clutch fluid changed along with brake fluid?
I highly doubt hot brake fluid affected the clutch.
It is something that gets missed during servicing. Not to mention it is a PITA on a lot of Front Drive French cars.

No idea about maintenance, but he said all worked fine until things got hot during the sessions.

Yeah, weird. I can't see hot brake fluid entering the clutch system from distant hot brake calipers. His issue might have been something different.

4 or 5.1
Whichever in in your budget

"Super" or "plus" DOT 4 brake fluid isn't really a higher specification. It's a viscosity which is more appropriate for ABS systems.

DOT4 is supposed to degrade certain types of rubber hose and seals.

With the combined system clutch and brake reservoir, I assume Mazda chose DOT3 to avoid any potential problems.

If you check the Castrol DOT4 brake fluids they both say "compatible with all brake fluids", so I assume the formulations have improved over time.

Nick.

https://bobistheoilguy.com/forums/threa ... st-4137864
https://bobistheoilguy.com/forums/threa ... ids.200874

https://www.castrol.com/en_au/australia ... rake-fluid
https://www.castrol.com/en_au/australia ... fluid-dot4

nd55 wrote:DOT4 is supposed to degrade certain types of rubber hose and seals.

I haven't heard that, but while Dot 4 is for higher temps, it also absorbs water at a faster rate than 3. That's probably another reason manufactures use 3 for road cars which may not get the fluid changed regularly.