If your pipe plug is 1/4" npt, I would get a short ****** in 1/4". Then install a 1/4 to 3/8" bushing. Next a locking 3/8" ball valve and finally a 3/8" 90 street elbow.
I did mine all in 1/4" and think it would probable flow a little better if i would have done what i wrote above. I put a pan under mine and come back in a few hours. Also notice how I use a ziptie to lock the locking ball valve. Without the ziptie it is easier to open and the lock will rattle as the comp runs.
It might be 1/8" pipe thread too. Mine is 1/8". The ball valve makes oil changes so simple and mess free that there is zero excuse not to change the oil!
I have one of those compressor driven oil suckers. I make sure the compressor is OFF and stick the sucker tube down the fill hole. I found that not all the oil will drain out the regular drain hole....