I've watched that the few runs on the leaderboards has been done with the japanese version. Is there any kind of difference with the american release or why does the J version is used??

Personally, I just played the Japanese version because I felt like it. I don't know of any significant differences, though I never really tried to find them

Was verifying a run and trying to understand the differences as well. At the very least the JP title screen says 1991, and the US says 1992.