I use MTL or a 50/50 mix of MTL and MT-90.
I've had that combo in every Mazda I've owned.

 

I was using MTL when #2 shattered, it's only a 75W80 fluid

IMO the problem is the trans oil getting too hot for the viscosity, so my recommendation is heavier fluids. It may come down to how hard you push it as the exhaust can transmit a lot of additional heat to the transmission.

 

Yeah, heat is the issue.
Modern trannies don't have much in the way of "impact" forces against friction components, so the oil is mostly for cooling and radial lubrication.
Thinner oils dissipate heat a little more quickly than thicker oils do, but you may be giving up some lubricity for that advantage.

 

maybe need to measure the trans oil temp---someone hook it up!
OD