There are several I can think of that are. Generally without issue. Though I personally don't.
To my ears the gear boxes are a bit noisier with motor oil rather than gear oil. Could be my imagination of course. But my ears convince me otherwise. Know of two cases (I was one of them) where clutch plates and synthetic engine oil didn't play well together either. For what it's worth a good automatic transmission fluid is an inexpensive and effective alternative for use inside the primary.
Personally I don't use engine oil anywhere but the engine. It's just not a big deal to keep the extra fluids around. So using a fluid more accurately dedicated to the tasks they're doing is too small a hassle not to do. Having said that, however, there's another reason that I like every bit as much for using the different fluids in different places.
Engine oil in the engine, ATF in the primary and Redline in the tranny. They all look, feel, smell and taste different. Obvious differentiation between them. So when/if I ever have a leak I know what it is. Don't have to worry about being fooled because a fluid ran down off something else other than its original source. Self-diagnostic leaks are a wonderful way to be lazy.