If I'm not mistaken, motor oil actually lubricates better than gear oil. Thus, using it should not damage your gears. It will, however, cause clutch problems. I think the extra lubrication makes it hard for the clutch to grab. Thus the clutch starts to slip and get really hot.
That said, I don't know if using ATF (which I presume is even less slippery than gear oil) damages gears or improves the way the clutch works.
Also, I thought I read somewhere that the CRF has two separate oil cases (one for piston, etc. and one for gears), but that Honda recommends using the same oil in each one.