What i sent to Red Line:

>Differential Size/Type : Limited Slip 3.9:1 gear ratio
>Transmission Type: 5-speed manual
>Issues : I am trying to smooth out synchronizer functionability and
>reduce gear grind in my transmission. Also, I want to put the best
>oil i can buy in my differential.
>Recommendations : Can I use your 75-90 gear oil in both my
>transmission and my limited-slip differential (L.S.D.)?
>Mazda calls for 75-90 in both, but I read that the your synthetic
>75-90 gear oil is for use in racing L.S.D., and not sure if i can
>use in a non-racing application.


this is what Red Line sent back to me:

Zach,

Thank you for contacting Red Line Oil, in your RX-7 transmission I
would recommend the MT-90 in the transmission and the 75W90 in the
differential.The 75W90 contains a friction modifier so could be too
slippery for the synchros in the transmission, Mazda calls for either
a GL-4 or GL-5 75W90 in the transmission and a GL-5 in the
differential. The 75W90 can be used in a racing differential but a
dedicated track car would generally use one of our other gear oils.

Regards, Dave
Red Line Oil

should I do what he said? or use the 75-90 in both?

 

Dude, Redline might be a good product, I am not sure, I have never tried it. But here is some real world advice. I read another thread here where a bunch of users said that using the Penzoil Synchromesh helps alot. I tried it, as the fluids in the ol' RX needed changing. My first and second synchro was real rough, ground bad, but afterward it smoothed out enough that if I am not shifting real quick it will not grind at all.

 

I have the MT-90 in my tranny...out standing .... simply outstanding!

Edit - I am impressed with Redline's response to your question. Very nice!

 

awesome, i will try the MT-90

 

and, has anybody used this: Watter Wetter?

http://www.redlineoil.com/products_c...p?productID=53

 

mt-90 here too, good stuff for the trans

 

i heard that sychromesh is better in the trans, but have never tried either.just take everything w/ a grain of salt

 

I use the water wetter and no complaints. It works. Seemed to work better on my old civic's B16a though.

 

mt-90 all the way, i tried royal purple 75w90 and it just wasnt cutting it, when i changed my clutch and used the redline, butter, simply butter

 

You want to use the MT90, it's specifically meant to help with worn sync rings.

 

Oh, I too use the Redline, MT-90 tranny fluid, and not a complaint!

 

Indeed. I just got a new (very used) tranny that wouldn't shift into 2nd cold without double clutching. Now, only a little grinding!

And I go with what the guy from Redline oil says about using Redline oil.

 

Yep, Watter wetter is awesome stuff. just recently tried it. My 86 GXL was running hot...too hot, and id replaced everything but the radiator so far, and really didnt wanna replace that too (since im just getting it running so it can be sold) put the bottle of watter wetter in, noticable difference, engine dosnt heat up nearly as much. best 8 bucks i spent getting the damn thing to stop over-heating.

Put it in my 90 (since the temp gauge runs almost half way up) also made a difference. Im putting watter wetter in all of my cars from now on.

 

I am very happy with redline mt-90 and with the 75w90 in the diff.

 

Another plug for watter wetter. I used it in my 88 Camaro with great success. As soon as I figure out where I'm leaking coolant from, I'll run it in my TII.