Originally Posted by DeafM5

Upon inspection, they did only filled the transmission while the engine was off. That's why the previous owner could not add more oil. I drained from transmission and it only drained 3.5 liter of oil, maybe less.

I did this first time after transmission swap, then I realized that transmission pump will suck in more oil if the engine is running. I proceeded to add oil until it dripped out of filler. It has been fine since.