So I have a 2004 Miata built for NASA ST6. The transmission shifts beautifully except in long races when it gets really hot it then it starts to grind. I have had the same issue with all of the following:

Mix of Redline MTL and MT90
Mix of Redline MTL and Lightweight Shockproof
Ford XT-M5-QS Full Synthetic

I tend to think it is a heat issue. Again cold or warm it's great but super hot 30+ minutes into a race it gets grindy. I have been considering going to the Redline Heavy Shockproof or 50/50 mix of that and something else.

EDIT - I ended up adding a ton of heat shielding and then straight redline MTL and that seemed to work fine even for long races.

 

Amsoil MTG has been great for me.

 

I would second the Amsoil MTG. Also might be worth throwing a temp sensor in the 6 speed to watch what it is getting up to. There is a thread with a link to the part number for the fitting required.

 

You've done all you can in terms of heat shielding the exhaust side of the gearbox presumably?

 

Stock exhaust has heat shielding protecting the transmission. I'm sure he does too, especially if he knows it has a problem operating when it is hot.

 

I recently switched from Redline to Amsoil in my 6 spd and it improved the quality of the shifts.

 

Thats all well and good but racing is a different animal to stock and thats is why I said "presumably". Still doesn't hurt to double check the basics right?

For example, we heat wrapped the exhaust around the gearbox area AND rigged up aluminium heat shiedling around/beside the exhaust in the gearbox area to try and help prevent the 6 speed failures we were getting.

It didn't help the failures :( ..... but im pretty confident we reduced heat transfer in the process.

Im not entirely sure after trying all those usually well regarded oils that it is going to simply be a different brand of oil type solution, but hope it is, being the easier one.

 

Thinking its time for a cooler.
Not a points hit so why not?
Both your trans and rear would thank you.

 

Done several 3hr races with redline and Amsoil in the 6 speed. (2860rs @12 psi) both became resistant to very quick shifts after 1hr point. Needs cooler. Open high speed airflow bottom of car, very much doubt effectiveness ofheat shielding .

 

Miata AZ6 transmissions get hot (300°). Run a cooler particularly if you are F/I or doing enduros.

 

First bandaid would be improving exhaust heat shielding, unless you have done all you can. Second bandaid is fluid type and visocity. Have had better success with Amsoil and Motul for trannies versus Redline. You need a thicker oil.

My experience is that a much thick Motul fluid works better than thick Redline for the weights you would be using. I have heard good things about Amsoil as well, it seems to be sinilar quality to the Motul stuff when I've used it. Transmissions I have used wouldn't shift with thick Redline fluid when not hot

However, thicker transmission fluid is still just a bandaid. I've seen people run simple electric pumps wired to an in cabin switch with an oil cooler just for the transmission. Lack of a thermostat makes it dirt simple. You can run everything in the rear end of the vehicle and improve weight distribution. Even in a poor airflow location for the tranny cooler it will significantly reduce temperatures. Extra oil capacity is always good as well. I've seen people run them in the trunk with a small fan and a port cut out by the license plate.

 

We have been using Amsoil MTG successfully for about a decade but are now experimenting with Motul 300v in an effort to increase film strength even further.

Redline MTL is fine for daily driven street cars but has nowhere near the viscosity required for any performance he was really. We use empty out for autocross cars that have notchy gearboxes when cold. It's that thin. MT90 only a slight improvement. Heavy shockproof has film strength but does not have very good synchro bite. The Ford synthetic also too thin when it gets hot. Crunchy syncro time.

 

My experience was ford worked great until it got too hot. If I did it again, I'd put a good cooler on the transmission, I bet heat was the primary reason for problems and cooling would have helped a lot. Amsoil was worse for daily but better when hot compared to the ford.