If I were living in Japan I'd get myself a nistune and nismo sprung button clutch.
Or you know. Don't listen to us just do what you want. It's not my car.
Before you even worry about a clutch go and get a nistune instead of that tomei whatever.
You can't just go and put some random ecu in and expect the tune to be spot on for your motor.
So your brother got under the car and undid the fill hole to the gearbox And saw it was dripping out clean?
Only taking the Piss. Time to change it. Might as well do the diff while your under there
Change oil. Come back with results.
All this sounds like is your oil is shit or you are changing gears weird. I suppose you are still a novice and with time will come experience.
This happened to me a year or so ago as well.
Streaks have not always been there.
The streaks are easily manipulated by a cloth.
Lol I'm not getting a bonnet resprayed for this.
Still going with fixes