So I’m back with something maybe relevant here from my AI. I asked it what the safe operating range is for the Idemitsu WS fluid I’m running now and I think the answer may be of interest:
“The lab specs on Idemitsu TLS-LV (the WS-spec fluid) bracket a much wider range than your transmission will ever see: pour point of -54°C and Brookfield viscosity at -40°C of 6,900  on the cold end, and flash point around 170°C / 338°F on the hot end. Neither is your practical limit.
What actually matters in the truck:
• 160–200°F — normal operating range. Where the A760F lives on a typical drive.
• 200–220°F — hot but acceptable. Towing, sustained grades, stop-and-go in August. Fluid life shortens but nothing is being damaged.
• 220–240°F — caution. Oxidation accelerates sharply here. Rule of thumb: every 20°F above about 175°F roughly halves remaining fluid life.
• 250°F+ — damage territory. Seals harden, clutch friction material starts to glaze, varnish forms.
• 275°F+ — you’re cooking clutches. This is where transmissions die.
Idemitsu specifically claims strong oxidation and thermal breakdown resistance, plus seal conditioning that reduces the risk of leaks from seal shrinkage and hardening — that last part is relevant to you given the reseal work from the AT205.
The number worth watching: if you’re ever pulling a trailer in summer heat, log the ATF temp PID on a long run. Sustained readings above 220°F would be the argument for an auxiliary cooler. The GX’s factory cooling is adequate for unloaded driving but has limited margin under load in a hot climate.”
“The lab specs on Idemitsu TLS-LV (the WS-spec fluid) bracket a much wider range than your transmission will ever see: pour point of -54°C and Brookfield viscosity at -40°C of 6,900  on the cold end, and flash point around 170°C / 338°F on the hot end. Neither is your practical limit.
What actually matters in the truck:
• 160–200°F — normal operating range. Where the A760F lives on a typical drive.
• 200–220°F — hot but acceptable. Towing, sustained grades, stop-and-go in August. Fluid life shortens but nothing is being damaged.
• 220–240°F — caution. Oxidation accelerates sharply here. Rule of thumb: every 20°F above about 175°F roughly halves remaining fluid life.
• 250°F+ — damage territory. Seals harden, clutch friction material starts to glaze, varnish forms.
• 275°F+ — you’re cooking clutches. This is where transmissions die.
Idemitsu specifically claims strong oxidation and thermal breakdown resistance, plus seal conditioning that reduces the risk of leaks from seal shrinkage and hardening — that last part is relevant to you given the reseal work from the AT205.
The number worth watching: if you’re ever pulling a trailer in summer heat, log the ATF temp PID on a long run. Sustained readings above 220°F would be the argument for an auxiliary cooler. The GX’s factory cooling is adequate for unloaded driving but has limited margin under load in a hot climate.”
