mx5cartalk.com

This gearbox has had a hard life. It takes about 1/2 sec to feel it into 3rd without a crunch and 1 or 2 secs to feel it into 5th. The other gears are a fair bit better but not perfect. I have changed gearboxes so many times on this car that the fun has really gone out of it.

It's currently running Redline MT90 which I believe (rightly or wrongly) is a bit on the slippery side. I think a higher friction oil may help the synchros do their job.

Am I on the right track and does anyone have any recommendations for a better oil than the MT90 for this box?

I'm using Nulon GL4 in the NC and it works a treat with no cold baulking into 2nd etc.
https://www.nulon.com.au/products/gear-and-differential-oils/full-synthetic-75w-85-smooth-shift-manual-gearbox-and-transaxle-oil

I have the same Nulon oil in my 5 speed which works great but, that said, the gearbox wasn’t giving any trouble beforehand - so I don’t know if that’s much help.

Amsoil MTG is the best I've used but it's hard to get... recently I've been using the Penrite Pro Gear 75W-85 Full synth but I think there is a larger range including a GL-4 75W-90 that looks pretty good.

BTW even with the best oils my 6 speed grinds a bit hitting 5th. I just don't think these gearboxes like fast shifts at high rpms... The synchros just don't get time to slow the gears enough. If I take my time it's fine but anything fast is crunchy... more so when they get hot.

With worn synchros, I can't see any of the suggested oils being any better than MT90.

Try Redline Lightweight Shockproof https://www.redlineoil.com/lightweight-shockproof

The problem is that we've all used nothing but premium low-slip oils in high-performance situations. I'm beginning to wonder if budget Valvoline from Kmart might work better in a dodgy old transmission. I realise that ravens might come and pick out my liver if I use anything except a fully synthetic oil, so maybe I'll try the Nylon or Penrite before I go that far.

MT90 is great for high heat situations, Lightweight Shockproof is great for old worn boxes. I’ve run 1/2 and 1/2 in my track car for a few years and it really helped with smoother shifts. I’ve just changed my gearbox oil and had 500mls of the lightweight left so mixed it with Penrite Pro Gear 75w85, this worked great as well. I couldn’t find anyone that stocked the 75w90 GL4.

The Americans swear by Ford Motorcraft transmission oil, over MT90 even, but I haven't been able to find it locally

in my NB 6 Speed I went from 100% Lightweight Shockproof to 100% MT90 in my 6 speed and it went crunchy instantly (only when cold though but never had an issue before).
Changed to 50/50 MT90 and LW Shockproof which is recommended by Fly'n Miata and has been brilliant, from cold mornings to hot track days not a hint of crunch and super smooth changes.

I'm using AMSOIL 75-90 GL-4 with no issues. I have another brand of 75-90 GL-4 that I will be testing soon.

Castrol VMX-M here, or whatever they changed the name to a couple of years ago.

Seems fine on my 5 speed with the dodgy bearing in 2nd.

Been running Redline Superlight ShockProof in GB since 2006 - GB has been fine & was fine prior to the Redline. Clutch needed replacement recently, but mech also replaced GB oil with a basic Penrite GB oil without advising he would do so - GB is fine hot, but cold it's not, so need to change oil again.

manga_blue wrote:I'm beginning to wonder if budget Valvoline from Kmart might work better in a dodgy old transmission.

Phil, interestingly (I think), many including myself run a Dexron III automatic transmission fluid in their Toyota T50 5 speed manual gearboxes.

Thanks guys. In summary each of all the good quality oils we all already know about work pretty well in somebody's gearbox.

I've been doing some research as well. Co-efficient of friction is key., but I don't really know if I need it to be more or less slippery. I dug up an old email from Redline when I asked about problems crunching into 3rd with Shockproof that said

Phil,

Thank you for contacting Red Line Oil, in your Miata transmission I would recommend the MT-90. Do you know what fluid is currently installed? It may be too slippery for the synchros.

Regards, Dave
Red Line Oil

I think I've determined that Dexron III ATF is more slippery than Redline Lightweight Shockproof which is more slippery than Redline MT90 which is more slippery than Redline Heavy Shockproof which is more slippery than the Nulon and Penrite GL-4s and they're all a helluva lot more slippery than generic dino 80W-90.

I have also found heaps of Penrite full synthetic ATF and a quart of very slightly used Redline Lightweight Shockproof in the shed. So any playing with more slippery mixes does not involve any cash.

The plan is first to try more slippery mixes by progressively mixing in Shockproof or ATF. If that seems to be taking it in the wrong direction then I'll buy full synthetic GL-4 Nulon or Penrite and start playing around with mixes of that with MT90 or Shockproof or, worst case, dino oil.

I may be a while. First priority is to head out the back of my local state forest to section up some ginormous red ironbark hollow logs to make containers for orchids. I'm figuring 2 days to cut and get them home and 5 days to recover from dragging them out of the logging slash and hauling them uphill.