Ok I bit and went down the rabbit hole. Also known as the “giggle” machine.
CYA: The summarization of the regurgitated information here within is from a memory bank within my brain that was regularly subjected to large amounts of Budweiser and Whiskey in my youth due to a disagreement between my body and brain......
Conventional Fluid:
220F: Varnish Starts to form on frictions and hard parts which allow the clutches to slip more creating more heat. Vicious Cycle
240F: Seals Harden, Additives Cook
260F: Clutches begin to slip as fluid is breaking down rapidly
315F: Seals Clutches Burn out and Carbon is formed. The fluid is useless and will cook a trans in short order.
Synthetic Fluid can handle higher temps but couldn’t find stuff like the above. Basically found things saying 220F is fine and 250F is getting close to the danger zone.
Synthetic fluid is able to keep the viscosity up at higher temps so the pump can create proper pressure to avoid slippage.
There’s also flash temp vs prolonged temp.
OEM’s seem to mostly measure fluid temp in the pan or body of the trans. (Aisin) has an option to watch converter temps in the RAMs I guess. Seems OEM’s put the vehicle in limp mode after so many seconds of fluid temps ranging around 270F.
My experience: I have BOILED a 4l80e on FSR’s running 4-hi 2nd gear vs 4-lo 3rd gear. The trans was literally puking fluid from everywhere underneath (vent....?) (only rad cooler at the time). I have also been been put on the side of the road at 3am on a long climb on I-40 letting the computer control my vehicle (2007 Yukon Denali 6.2/6l80) when the XMAS tree lit up due to trans fluid temps. TH-400 has never failed me with only rad cooling, same with TH-350. I grew up GM.
I have been convinced that doing what
@Cubey does is best practice to measure both.
Run synthetic and lube guard if Post ‘86 Ford trans (website said that)
I would run a TC output sensor and put 250F as my prepare and 270F as my take action temp. And run a pan temp and expect 100F over ambient or cooler. Pop the hood and look/smell fluid once in awhile.
Since I’m short on time, I plan to do neither on my 4l80 (added external cooler) or E4OD and just keep smelling/looking at the fluid and change every 30-50k.
You guys hauling heavy or towing often, I also went down the rabbit hole of how hot the rear axle gets.....yikes!!
Please correct anything I misstated!!