I don't know much about oils, but doesn't ATF have detergents in it also? Isn't that why people put ATF in their engines to loosen sticky lifters?
Also believe it or not, Ford recommends motor oil in some of their old trannies in the 70's and early 80's. I thought they all took gear oil, but some do take motor oil.
The "foaming" with motor oil must be old school or something(I am old school also, and have filled old air compressors with non-detergent oil). Most all hydrostatic units in lawnmowers use detergent motor oil. I put Lucas 15w-40 diesel engine oil in the hydrostatic unit and the engine on a Dixie Chopper I service, that's what the dealer recommends to put in it.
A friend of mine bought a brand new skidsteer a couple of years ago(a big one) and guess what the hydraulics are filled with? You guessed it, the manual calls for multi-weight motor oil in the hydraulic system.
So I understand where some of you are coming from about the "non-detergent" to prevent foaming, but I think it must not be a problem since everything that likes to promote foaming, like hydraulic systems, are using it now.