In the morning today, I decided to change the engine oil myself so I opened the bolt and left the bike as is for an hour to completely drain out the engine oil, then I replaced with the new one. I must say the bike now becomes more responsive and smooth, gear shifting has also improved a bit.
The purpose of sharing this experience is that, last time when I got the oil changed from the mechanic, they don't wait for the draining process to completely finished and replaced it like in 10 minutes. There was still a lot of oil left in the engine and I personally didn't observe good results after replacing the oil, like I did in the morning today.