Can I replace my transmission fliud with motor oil? Is that a good idea or get the genuine honda oil? Will the motor oil cause any damage to the transmission?

 

If it's a manual transmission, 10W-30 motor oil is fine. Of course the OEM recommends you use their MTF - why wouldn't they?

At most you may be missing out on some friction modifiers - not too much to worry over.

 

You can run motor oil. But for the extra few bucks get what was made for your transmission.