there are two reasons for this: the gearloading is usually higher and ATF,s are far thinner (and include VI-improvers) so they could not possibly meet the higher requirements for shear stability.
Thanks romke. From a hardware standpoint I don't understand why the same model cars, one with an AT and the other with a MT, the MT one would have higher gear loading to make it more shear severe? Conversely, what is it about AT's that keeps them from shearing the VII's in their fluids that can't survive in a MT? AT sumps are getting smaller but still have typical fills over 4L; how much fluid do passenger car MTs take, and how much of a role does this play in the shear severity?
the higher loading with a standard gearbox is the result of the design of the gears. in automatics usually planetary gears are used and due to the fact that the planets and the sun gear have to fit into a ring gear, the loading in terms of pressure per surface aerea tends to be a lot lower. another reason to keep pressures lower is the fact that the lubricant used has a lower viscosity, which is necessary for the hydraulic operating system to function at very low temperatures. due to the lower loading of the gears also the shearing forces are lower.
the amount of fluid also plays a role, in an automatic most of the fluid goes into the torqueconverter, but ofcourse a larger amount of flluid is circulated and therefore the "shearrate per volume" is far lower then in a manual gearbox that contains usually somewhere between .45 and 1.5 litres of oil.
