I have a Turbo II trans, and am struggling to find which gear oil is best for it. This transmission is in near-perfect condition and it would be a shame for me to be the one who messes it up after all these years. What is the best transmission oil?

I have read about the difference between GL-4 and GL-5 here: http://www.widman.biz/uploads/Transaxle_oil.pdfSecond, I have checked the Turbo II FSM, which specifies GL-4 OR GL-5 80W90. With the above information in hand, GL-4 seems like the better option. (Can anyone confirm if the TII synchros are made out of a harder metal?)

Unfortunately, many manufacturers don't label their product as GL-4 or GL-5, and almost never mention the composition or presence of S/P additives.

Royal Purple
I recall reading threads on RX7Club where plenty of people use Royal Purple 75W90. I have bought a few quarts of the stuff, and it says proudly on the bottle that it's safe for brass and therefore safe to use in synchromesh transmissions, BUT it does not mention if it's GL-4 or GL-5, and on top of this I cannot seem to buy quarts in 80W90, only 75W90.

Ford Motorcraft
Info from here: https://www.fordparts.com/Products/C...ionFluids.aspx
The Miata guys' hivemind hails Ford Motorcraft 75W90 MTF as the best gear oil, but with little justification. I have a strong understanding of chemistry and am not a fan of bullshit arguments. It too fails to specify if it is GL-4 or GL-5, and is slightly off from the OEM specified weight of 80W90. I am not yet convinced that this is significantly different from the Royal Purple.

There are others, too: Pennzoil synchromesh emphasizes its compatibility with soft yellow metals, but fails to even list its viscosity rating.

 

first, 80w90 seems to be impossible in the US (or even outside), 75W90 is the closest you can get, and its only 5w's off, which is nothing.

second, technique has more to do with transmission life than anything else. you should be able to shift with your fingertips.

for a street car, a 50/50 mix of ATF and whatever gear oil, works great. it shifts cold like synthetic, and its cheap and available everywhere. for a race car i tend to use redline, but pretty much any synthetic is good enough.

 

I second that I've been running 50/50 or 75/25 using ATF type F for the past few years .And always works great . GL5 is good for the diff but I would not use it in the transmission.

 

The FSM says 75w90 or 80w90 is fine:
http://www.rotaryheads.com/PDF/2nd_g...sion_Turbo.pdf

redline MT-90 is GL-4 and made for transmissions:
Red Line Synthetic Oil - Gear Oil for Manual Transmissions - MT-90 75W90 GL-4 Gear Oil

amsoil also makes a 75w90 for transmissions that is GL-4:
AMSOIL Manual Transmission & Transaxle Gear Lube 75W-90

 

i run synchromesh in my car, it is however louder because it is thinner than 75W-90 but shifts smoother and easier. as j9fd3s mentioned i also have done plenty of 50% blends of auto trans fluid and 80W-90 for manual transmissions with no ill effects. straight 75W-90 or 80W-90 will allow the transmission to last longer but at the cost of shifting ease.

i dont think i would recommend straight gear oil, maybe even as little as a 10% ATF mix. i have pulled apart some really dirty trannies before... i did put some 80W-90 in the exes RX8 and that tranny was a bit sluggish with the shifts when cold.

 

I'll say RedLine MT90 as I've used it since the car was about 6 months old. When the car was new,
weather cold, the trans couldn't be shifted, so Mazda was recommending ATF, but I decided to use RedLine.
Also RedLine in the diff.

 

i would only use expensive synthetics if you either

a) have too much money

or

b) race the car regularly


same goes for engine oil. if there really was a difference, why are there still arguments about it after decades of testing on this forum with very nearly zero definitive result. the main difference is in how long they take to break down and how much heat they tolerate before breaking down.

 

i'm currently using royal purple.... cant remember if 75w90 or 75w140.
been thinking of using 85w-something in it and see how it goes.
i do think gl-5 stuff is no good in the transmissions and think it causes damage to synchros... but then again i'm always revmatch downshifting.

do thicker oils such as 75-140 vs 75-90 make better heat transfer?

have used valvoline durablend 75-90
valvoline synthetic 75-90
synchromesh... didnt like it personally. i added it to royal purple oil ( are you supposed to do that), and it seemed to make it worse, could tell it is thin.
regular valvoline 80-90.

had a weird idea where limited slip additive may help get into gear with the synchros...?

have/currently used redline the blue stuff in a packed s4 t2 diff, no problems. thought it looks too thin to use in a trans though.

only my two cents.... as i have a spare t2 trans waiting in the garage...but then again drifting/mountain driving for 5-6 years on current t2 trans and no problems.

 

why would you think any gear oil is too thin for a manual transmission? you seem to have a heavy bias toward heavier is better when truth is, thinner makes shifting easier but at the cost of wear. heavier oil will allow the trans to take more abuse but you will need to put your foot on the shifter to move it.

 

ATF/gear oil mix.. Been using this in all of my Mazda gearbox's for years now including my '88 Rx7. And FWD boxes for that matter.

 

I'm curious about this ATF/ Gear oil mix. Does it help or hurt shifting when a syncro is about shot? Prolong or reduce the remaining life in said situation. My 3rd gear upshifts ok but a little chunky sometimes when hot. Cannot downshift without heel/toe and rev matching but is smooth if you get it right. Other gears are ok. Currently has Royal Purple that's way past due for a change. Like there's over 100,000 miles on this oil. Trans has approx.28x,xxxmi. on it. It still does better than the Mobil1 I had in there before IIRC. Anyway was gonna use more Royal purple but this mix has my curiosity peaked.

Edit:
The weight is 75w90.

 

Thank you for all the replies. I am sure this thread will be valuable for others in the future. Looks like Redline is a good move if you want to follow Mazda's instructions to a T. I ended up getting Lucas 80W90 Transmission oil (GL-unknown) because that's what the auto parts store had and it's probably not healthy to overthink this too much.

 

well said.No sense in stressing over a simple "fill and forget" maintenance item.
I started off 10 years ago with this stuff and just got Redline.
I put it in all the cars I have had.
The only regret I have had is when I sell the car the trans has "money in it"..lol!

 

I put a big heavy turbo box in my Mx6 (FWD)about six years ago after going through two auto box's and two diffs in the smaller, much lighter N/A box's.

Second was very crunchy and very notchy and even matching revs it would still crunch into gear. It was like this for about six months and was only getting worst to the point the selector arm on the end of the shaft sheered the bolt. I thought it was dead but I needed the car and couldn't find a box. So after pulling the box down and fixing the sheered bolt, I remembered what we used to do with the old Rx4 four speeds back in the nineties.... ATF. That box is still going today and hasn't been pulled out since, running a mix of ATF (mainly) and gear oil.

 

True. I spent most of my time the past two years setting up the rear end of my car including all suspension, bushings, links, and diff settings. I spent alot of time and tried alot of different differential setups, and came to a conclusion to use heavier oil in the diff, so i guess my ideas transferred to the transmission as well.

after adding the synchromesh to the royal purple, i had issues not being able to shift as well, which probably led me to believe that the thinner fluid made it more difficult to shift... not provide enough lubrication? it's probably all in my head, but that's what i experienced so my mind was made up.

I really do enjoy threads like these. I had a talk with a oldschool hot-rod drag guy who gave me a big speal on engine oil viscosity and how too thick of a oil is actually extremely bad for the engine since excess pressure is exhausted by the oil press. regulator. and how a thinner oil will actually flow more oil through the engine resulting in more cooling, and the film thickness is enough to protect the motor.

This doesn't transfer completely to transmission oil, but I wonder about the film thickness issue vs. temperature.

BTW: had to rebuild a ford ranger man. trans. a year ago. gears stripped out from wrong atf being used ( used atf from factory). seeing the results scared me from potential wear of the gears. (again... nothing against using atf in transmissions)

 

I'll throw another non-RX7 in the mix here to help with, or cloud, the issue. I DD an Acura DC5 (RSX-S for those that don't know) and they have notoriously problematic six speed manual transmissions. Amsoil synchromesh has been just awesome in mine and its been in there for a few years. I had gear grinding problems, difficult to get into gear, 3rd gear pop-out (it would just kick out of third gear after you were in it and driving for no apparent reason).

Shifts like butter now.

As far as the thicker/thinner thing, it's not like Honda manual trans fluid is all that thick. Whatever Amsoil does different works very well in those transmissions and I think the only difference between the others mentioned above is that the synchros are carbon instead of bronze.

 

there is always a sweet spot for most any transmission. keep in mind syncros tend to work best with little to no lubrication but they sacrifice their soft metal for the shift so adding a hypoid gear oil will help slow down the wear significantly and the ATF acts as a thinner to ease shift quality.

i may have to do something with my honda insight transmission because it really hates downshifts with the OE fluid. i feel like an idiot grinding gears when i know how to drive just fine, even rev matching doesn't help in that trannies case... i really don't feel like disassembling the transmission just to mod the syncros even though that is the apparent fix for it.

 

A coworker that used to race a FC in SSCA got me into the redline shockproof, switched from MT-90 to their shockproof, and never went back.

 

Currently using red line mt-90. Downshifts are not very friendly to say the least. I may try the atf blend when I drain the trans in the spring.