Small tight tranny cases with small cavities/passageways/ports require light fluids to allow full circulation. While the motor oil doesn't have the loading capabilities of gear oil, the small loads, relatively speaking, of these four cylinder engines permit the use of it. Plus motor oil will transfer heat a bit better.

Acid is primarily created by moisture in the case, which enters through humid air being drawn in by the cool down process at the end of each use(or it was driven through a creek!). Not a problem if the fluid is changed like it is supposed too and driven frequentlly but if vehicle maint. gets neglected for 10 years and it is driven infrequently during that time, you will see some etching beginning on tranny/differential components. Very common is larger hotter running cases like the tranny cases and diff cases of my tractors.

NCDiesel