A lot of essential oils would irritate mucous membranes and would not be for internal use. I know slathering an oil all over your dick would probably not cause any irritation (except for maybe the head), but I imagine your lady friend might find it an irritant.
Bio Oil has a rose oil base and contains Vits E and C and delightful on dick and balls. Also you can use for shave oil (including if you shave balls and/or dick) and aftershave.