Effect of viscosity on auto trans shift?

[strat81]

What affect does fluid viscosity have on the shift feel/quality of a modern automatic transmission (torque converter type, not CVT or dual-clutch)?

How sensitive are modern transmissions to small changes in viscosity?

If possible, assume all else is equal (i.e., friction modifiers and other additives are all the same).

 

[Pablo]

Small changes?

None. Think about it. 10°F fluid or 200°F, tranny shifts the same.

 

[Hokiefyd]

Same discussion, from a little over a year ago:

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2663400

 

[440Magnum]

Mechanically, not much. But the computer in some cars runs radically different shift programs when the trans is cold than they do when its hot.

 

[rslifkin]

My Jeep seems to shift softer with cold, thick fluid than with warm, thin fluid. The computer can't control line pressure or shift firmness, only shift rpm, so it's definitely a mechanical difference.

 

[andyd]

On the old school 727 Torque flights and the 4 hp22s I'm used to, I could tell if the fluid was low by how it shifted from low to two. Like a slow, ride the clutch pedal out, kinda shift. Smoov! It was just the first cold shift of the day. Check the stick and it was a pint low.