I'll tell you what, you put motor oil in your tranny, and I'll put purpose made synthetic tranny fluid in mine. I'll enjoy my effortless shifts and decreased syncro wear and you can enjoy notchy shifts and syncros that are on the verge of becoming molten due to a lack of friction modifiers. You only need 3 quarts. Don't be cheap. You are looking at $20 for Redline, Amsoil, or Royal Purple MTF or hundreds in repairs because you decided to take a risk. Is it really worth it? Leave the stock fluid, or get something better. Motor oil, no matter how good, is a downgrade. It is a bad idea. I repeat, BAD IDEA.
Motor oil contains all kinds of additives that don't need to be in a tranny. How much fuel dillution happens in a transmission? How about acids from sulfur and nitrogen? Oh yeah, that's right, there is no combustion in a transmission. Tranny fluid can also use tons of anti-wear additives that motor oil can't because of emissions and such. Use a purpose made fluid, I will say no more. End of discussion.