Originally Posted by
loepke72
I think it has to do with that difficult 3-5 shift, and maybe not wanting to do that with cold fluid and screwing up the precise timing needed.
That is
exactly correct! When the ATF is below 0°F we could not get the timing on the 3-5 shift to work correctly. It was MUCH easier to shift 3-4. The 3-4 shift is non-synchronous, all that happens on that shift is the overdrive clutch applies and the overdrive one way clutch overruns. If the clutch takes longer to apply all that happens is the one way clutch holds for a bit longer. The driver won't notice anything wrong.
The engine runs 9% faster in the fourth than fifth at the same vehicle speed. This has the added benefit of helping both the engine and trans warm up faster.