**Tranny & Diff Fluid** What do you recommend?
 

Hi guys,

I have been owning my FD for 5 years now, but I have not yet tried to change the transmission and differential fluid myself (had them done by mechanics though) :blush: So this time I am going to do those myself.

I did my research about what BRAND & MODEL fluid to use for FD's transmission and differential, but all I could find were either older threads (6 years ago) or debates that did not come up with a conclusion...

I notice that for transmission, some people recommend REDLINE MT90 75W-90, some other recommend PENZOIL SYNCHROMESH, and some other recommend just conventional GL-4 75W-90. As for differential, some people recommend GL-4, while some others recommend GL-5.

My car is currently making 320WHP, stock LSD, with the exception of 4.77 final gear. It is mainly street driven with occasional track days. What do you recommend based on your experience?

Mike:

Attachment 608877

Been running them for 3 years now, tran shifts smooth and no issue with the diff.

I wonder what the shelf life of neo fluid is...

+1 on Neo. I get mine from Mazdatrix.

Guys, thanks for the input. I think many of the forum members here debated about the synthetic oil eating away the yellow mental (synchro)?

Also do you run just one oil (like the NEO 75w-90 HD, or is it NEO 75w-90 RHD?) in both the transmission and rear differential?

Both take 75w-90

I've used Amsoil 75w-90.

But any top-shelf brands of gear oil will do.

If you have an aftermarket LSD, you'll want to use their own oil and additive if they require it.

I've tried syncromesh in the transmission and while it makes it shift super slick, i prefer the notchy feeling of a well lubed bolt-action rifle. Amsoil gives me a buttery smooth engagement, but a nice positive feel.

You're not likely to see a conclusion here either. Like asking what brand of beer is the best. But FWIW, +2 on NEO HD 75w-90.

Whatever you choose, since this is your first time, it's really pretty simple but seems many get hung up on the transmission. IIRC it's in the FSM but remember, ONE fill (A) and TWO drains (B & C)....leave the other plugs alone and don't "drill" the other plugs in too tight. I've seen them almost thru the trans. case. I also use a little Teflon tape.

Attachment 608860

i used royal purple in the transmission and diff on both my previous FDs (and did a lot of tracking) and it was fine. i'll be doing that again with my new FD.

I used to recommend Redline MT90 instead of MTL back when I was tracking the car. I think it still makes sense on a car that gets tracked heavily since the thicker oil won't break down as fast as something like MTL. However, it takes longer to get the MT90 up to temp and shifting smoothly. Time which results in wear and mediocre shifting til it gets warm.

For a street driven car, I'd say the MTL is OK but I recently started using the Amsoil 75/90 on my Spec Miata and it shifted better than the MT90 I had been using. I'll probably be trying the Amsoil in the tranny of the FD soon.

For the rear, just get the Redline gear oil in 75/90 NS. Remember to get the NS as the Torsen doesn't like or need the LSD additive.


I've read too many failures on Royal Purple gear oil so I say no to that brand.

:werd:

Circle Track transmission Oils 75w90HD 75w90rhd LW18 Neo Gear oil synthetic
FWIW Baker Precision carries the full line of NEO stuff and $2 cheaper per quart than Mazdatrix on 75w-90 HD.

^ Yep, that's where I order mine.

Really appreciate all your inputs guys. I think this time I am going to try NEO 75W-90 in my tranny and differential. Next time I will try Amsoil and RedLine and will post up a comparison

I heard that I HAVE to remove A before I remove B and C or A will be stuck? Also what do you use to remove C if I don't have 24 mm socket handy? Do you think that a monkey wrench will work?

Removing the fill plug (A) first is the smart way to do it. Just in case there's some difficulty in removing it later you'll know it before the gear oil is drained and you can't get any back in. But has no direct affect on the others.
Don't recall the size of that large drain plug (C), but 24mm sounds right. Buy or borrow the appropriate socket (there might be a SAE equivalent...15/16 maybe). No, a Crescent wrench won't work, a socket is needed but it shouldn't be on that tight.

^You don't necessarily need a socket. I bought a 23 and 24mm combination wrench for the diff and trans drain and fill plugs. I had a difficult time getting a socket to fit on the plugs on the rear diff due to space constraints iirc.

https://www.craftsman.com/products/c...pt-combination

Great thread guys. I've always preferred Neo as well. :icon_tup:

One thing that I try like hell to avoid using on my FD is a monkey wrench. Just sayin' :D

Seems like whatever the OP chooses, if it smells like a camel's ass it's probably gonna be pretty good stuff. Synthetic gear oil just smells nasty.

Huh, I thought the gear oil smelled nastier than the last camel's ass I smelled ;)