The rubber gasket was not installed properly and misaligned resulting in oil pan not seating properly. In fact, it managed to cut through small part of the rubber gasket causing more leakage. From the service history, it looked like this was done while this NSX was in Japan. ...
I carried out the TB service on this NSX recently and as the result, there was no more oil leakage from the valve cover gasket area. Thus, the surface of engine/cylinder block above the oil pan was very dry. However, the top section of the oil pan was already wet all the way round even after it was cleaned recently so it is definitely leaking from the pan gasket area. ...