I got a second hand Dowina accoustic guitar and I found the neck a little bit curved. So I tried to adjust the truss rod. The truss rod bolt is visible near the sound hole. But it was very difficult to move. I turned it a little bit counterclockwise, but with difficulty. I am a little bit afraid now of turning it further. Should I go on turning the nut that way to move it out, so I can lubricate it? Thank you very much for your opinion and insight!
PS: I add a picture of the guitar neck. Please, according to your experience, 1) is it already straight? 2) Should I file the nut a little bit lower?