I've used Revomax 80W90 from Pettit and Neo 75W90RHD ("RHD", not "HD" -- wtf is the difference?)

Neo 75W90HD and Redline MT-90 seem to be the most popular choices on here.

I noticed some difference from whatever came with the car when I switched to Revomax, and again an improvement with Neo 75W90RHD. I still get some knick on 3rd gear though, and of course 5th needs to be rev matched every time.

I'd like to try MT-90 next, but am also curious about MTL (75W80) and MT-85 (75W85). Is any particular viscosity known to be better for synchronizers? Is putting 75W80 into the tranny a bad idea?

I used MTL on a FWD Mazda MX-3 years ago and it was absolutely night and day better than anything else. Apparently that tranny was supposed to use GL-4 80W90.

 

I put MT-90 (Red Line 75w-90 GL-4) in my transmission, and I still get those nicks in 3rd and 5th you described.

I have no complaints, but it might not hide your synchro issues.

 

If you don’t drive your car when it’s colder than 50 F. then 80w-90 is fine according to the FSM. Otherwise 75w-90 GL-4 or 5 is specified. I would not go thinner.
Seems like the biggest difference reported by most is how the various choices in oils perform over the operating temperature range. Some seem to lead to a little notchiness when cold but smooth out once warmed up...or vise versa. For me this is noticed most during 1-2 shift.
I also asked what the difference was between the two NEO flavors. I was just going to call them but haven’t gotten around to it.

Never had the need to rev-match 4-5.

 

there is a guy Barry Hartzel, in Seaside who will sell you Swepco, and tell you that the synchros like it more. in the miata transmission this seems to be true, have not tried it in an R box. Swepco is actually a non synthetic.

it seems like there are three different requirements for a transmission oil. Lubricating the bearings, gears, and synchros. the Swepco favors the synchros, and i think the other stuff does better with the bearings.

obviously i have no hard data on this, except that the local racers swear by the Swepco. this is also not meant to be an ad either, its just an option

 

Best shifting manual trans gear oil I have tried in the FD is CRC Sta-Lube 85w90. It is a GL-4 formulated to be "yellow metal" (bronze) safe. There are some gear oils out there that will damage synchros and bushings due to additive packages.

Available at your local NAPA for a reasonable price.

The info online on what modern gear lubes are safe/unsafe for an older trans is a little murky, so I stick with gear oil that explicitly says it is yellow metal safe.

We also ran the 75w90 limited slip formulation it in the rear end of the lemons car with good results.

 

So many opinions, so many options...

It sounds as if thinner oils are better for synchros (this would be consistent with my previous experience). I'd be willing to sacrifice a little bearing life for smooth shifting. This guy says he ran Redline MTL (75w80) for a long time and that it shifted well, but his bearings weren't so great when he did a rebuild: A newbie's adventures in racing: Transmission Oil Change & intervals That makes me tempted to split the difference with MT-85.

Apparently the Porche guys are in love with Swepco 201, but I can't seem to find any concrete feedback about it being used with RX-7s...

 

It seems like most car groups have a trans oil they dig - I know DSM guys were all about BG Synchroshift, for example.

In my experience a good synthetic will shift smoother than a non-synthetic. I'm currently running Idemitsu gear oil and I've been happy with it so far. But so much of this is VERY subjective - "felt better" "shifted smoother cold" etc.

Go with a quality synthetic and the appropriate weight and you'll be fine. Big thing too is to CHANGE IT, I think the change interval in the service manual is like 30,000 miles or something and this REALLY gets overlooked. I know I worked on FC's back in the day that I don't think EVER had gear oil changed.

Dale

 

all the local racers have run that stuff since the 90's, and they really love it. not really internet people though. in fact to buy the stuff you need to go to Barry Hartzels shop, and its expensive

 

Apparently the internet is catching up because Bezos has it for about $100/gal. I’ll call Hartzel and see how much he wants. Probably just buy it from him anyway

 

I’m not a racer, but guessing they’d want a gear oil that warms up quick and is as thin as they can get away with for less driveline loss. They also might be using a trans cooler. There also might be differences in the needs of straight-cut gears if being used. And they’re probably changing that oil more frequently than most of us.
Not sure going to a thinner viscosity, especially in a warm climate, would be the way to go on a street car for a little smoother shifting.

 

BTW not trying to sell you this oil, there are TONS of different ones, its just an option that works, and you're close enough to be able to get it

 

with the M box (MIata, NA Rx7, etc) the concern is to keep the synchros alive, its always the first thing to go in those, particularly 3rd gear. although you have a point, and in something like a spec miata, less drivetrain loss would be a big deal.

its why picking an oil is hard, there are conflicting requirements

 

fwiw, Myself and my buds use MT90 in Spec Miata. We are mid pack and too cheap and busy to run up front so we need the bearings to last more than we need the extra .5 HP of the thinner oils. The 90 weight takes a bit longer to warm up but not important for racing.
For the FD, the MT90 takes a bit longer than desired to warm up so you can scratch 2nd gear on shifts until the 90 weight oil warms up. If you live in a cold area or don't warm up the car much, MTL or MT85 might make sense to run but I wouldn't run MTL on a track FD. I mixed mTL and MT90 in my last trans oil change.

 

I called Neo yesterday and the receptionist didn't know anything about "HD" vs "RHD" other than "RHD is for racing." She gave me the owner's email -- still waiting to hear back from him.

Barry Hartzel sells Swepco 201 for $75/gallon.

Edit: Also learned that Barry teaches "Automotive Technology" as an adjunct professor at a local community college...

 

oh? that would be an interesting class

and while you're there take Gary's design class https://webreg.mpc.edu/SR_ScheduleOf...D&Course=23971
its really cool, he has a really nice shop space (and a really really nice FD), and you can sculpt in clay, or 3d print, or cut and weld...