Mobil 1 motor oil in a tranny?????

[84LX89GT]

the clutches will wear out no matter what the lubricant because they are engaged and disengaged frequently and will cause dust in the pan, but it usually takes 30,000 miles for any amount of "dust" to build up or harsh use for a decent amount of time (hard to say).
Automatic transmission fluid has detergents that help keep lubricant passages clean and is designed specifically for the clutches and bands in the transmission. The weight of oil and trans fluid is generally the same and i've heard that running trans fluid in an engine for a short amount of time will help de-sludge the engine and clean out the oil passages.
I personally would only run the approved trans fluid, or synthetic auto trans rated equivalent because they are formulated to not hurt the clutch/band material.

I see no reason to run motor oil in a transmission, chances are it will work for awhile but the consequences of changing to something that don't benefit the transmission in the least bit could be the cost of a transmission.