The garage I usually send the RX8 to filled the differential with GL4 transmission fluid instead of GL5 differential fluid. I've clocked about 1500 miles already(Yes I just realised the mistake they made) Would this cause significant harm?

 

GL 4 is not a problem. I used Redline GL4 and it was fine.

 

Doesn't GL4 lack certain additives that GL5 has? I have noticed that the automatic upshifts seem to take longer than usual. Is this a side effect of using GL4?

 

I know someone that accidentally did this and the diff blew up shortly after at a track event.

 

You probably have not done any damage since you caught it pretty early, but I would get the correct fluids in there ASAP. I am confused on one point, however. You wrote that they used the wrong diff fluid, but the transmission shifts funny now? That part doesn't make sense...

 

I happened to see a similar bottle of what the garage put into the diff at an autoparts store while doing some window shopping. I examined it only to find out that it was GL4. Then I realised that they had messed up.

I'm not sure if the weird shifting has to do with the wrong diff fluid being put in. It wasn't like that before the diff fluid was changed.

 

The diff oil should not have any impact on the function of the transmission unless something is seriously wrong.

This is good stuff. 1.5 quarts will do the trick.

Amazon.com: Red Line 57904 (75W90) Synthetic Gear Oil - 1 Quart: Automotive

 

GL-5 is for Diff ONLY (Limited Slip Diff)

GL-4 is for Manual 6 Speed Trans ONLY.

DO NOT PUT EITHER IN THE WRONG UNIT.

 

After putting in Redline's 75W90, the car seems to have gone back to normal. It might just be me but it definitely feels good to have the right fluids in the car. That garage put in 140WT GL4 tranny fluid in my diff. WTF!!!

 

After driving the car 100 miles with Redline 75W90 in the differential, that annoying jerk when shifting from 4th to 5th at 6000RPM has disappeared. In fact the car runs a lot smoother. I'm not sure why the shifts on the auto tranny were affected by the fluid in the differential but I can say that a GL4 fluid in the differential will adversely affect how the car runs.

 

Check the Mazda recommended fluid available at your local dealer. for the RX8, in their reference book, they call specifically for mazda 75w90 GL-5.

i know because i was just there. i didnt buy it because it is $20/qt, and I will instead get the mobil 1 75w90 gl4/gl5 fluid...

 

My Mazda dealer in Cincinnati told me they gave me 4 qts of bulk GL4 and it turns out it was GL5. I never used it because the density and flow properties I measured a work didn't seem right. I ended up returning the dealer MT fluid (turns out they only have 1 fluid 75W90 GL5 and GL4 certified) and getting a refund.

I did put MT-90 into my S2 and after 400 miles of back road driving to DGRR, it is = in performance to my oriignal MT fluid the dealer had put in (MT fluid had 45K miles on it). I recommend Redline MT-90 in the S2 with no notchiness as compared to the fluid that was in it.

 

If you like MT-90, you should try MT-L next time. It has a smoother feel in a street driven car, with none of the cold weather notchy-ness.

Edit: Be aware that MT-L is a 75W80 oil, which is thinner than Mazda's recommended 75W90 oil. Use it at your own risk.

.

 

The best GL4 manual trans fluid is Ford XT-M5-QS and you should be able to buy it at any Ford dealership.

 

I had already purchased the MT90 so I decided to use. Will take both recommendations for next time

 

This.

This stuff is magical in my S2 gearbox.

 

I personally would not recommend using Redline MTL fluid in an RX8 trans, especially the S1 trans.

 

Do you care to share any reasons for that...?

 

It's only 75W80 and too thin. The RX8 trans runs very hot so a thinner than spec oil is not a good choice.

 

^ True. It is thinner than what Mazda recommends, and I should probably be more careful about recommending something that falls outside of the recommended viscosity.

In practical terms, it provides enough lubrication. I know this from the time I have spent with spec Miata racers. Their objective, when selecting lubricants, is to use the thinnest oils possible, that still maintain adequate pressure. I have seen a lot of MTL used in Miata transmissions that are raced for years without incident.

So, I extrapolate the stuff to provide suitable lubrication properties for a street-driven RX-8, however I should know better than to make such a blanket statement. I will retract it above.

 

Love Redline gear lubes, but found that the MT-90 gave slow shifts, and got much slower at the temps I encounter in winter.

I tried different mixes of MT-90 and MTL, finally settling on about 30% MTL.

Nice crisp shifts, with just the slightest 'well oiled' resistance.

 

Yes, I refuse to use MT90 for that very reason, but why you'd do that rather than use a product that works properly for the application baffles me. I'd strongly recommend trying the Ford oil, which is 75W90 GL4, and also has quite a good rep for both street and race use in Mazda manual trans if you google it. It truly is amazing in the RX8 trans; hot, cold, street, and race. It's a bit pricy, but you can usually get a decent discount buying it by the case. And no monkeying around playing the blending game with multiple oils. But that's just my experience using dozens of different lighter, heavier, and exotic oils during the destruction of six S1 transmissions. Thankfully I moved to a different race class and can use the S2 trans now ...

 

Ford Motorcraft XT-M5-QS

 

Thanks for detailed explanation TeamRX8

 

Here's the spec sheet for the Ford MT (wasn't able to attach the PDFs): https://www.fcsdchemicalsandlubrican...ission%20Fluid