A number of guys have this problem.
One member here found that he cured his by allowing a better "seating" of the cap before installing the screws. Seems if you throw the thing in and bolt it down. It flexes and doesn't allow the O ring to really seal. He pressed it in and turned it back and fourth as if screwing it in and out a couple times. This better allowed the O ring to press into the bore sealing it and stopping the oil from passing by the seal and into the box....
Yea I seen that vid of him seating it better. I was one of the first people with this problem. There has to be a way to solve it.