Gearbox grind

Hello, I own a 2013 Qashqai 1.6 petrol with manual transmission since one year now. One week later when I bought I start noticing a 5th gear grindind every time a shift fast or when I have the car in neutral and then shift to 5th gear, the previuos owner told me he change the clutch. It really was not a big deal to me I get use pretty quick and have almost not grinding at all, now I am selling the car and the person that test drive it the first thing he notice was the grinding (Btw; he really was bad driving manual but yes the car has that small problem) he was really interested un purchasing the car because is pretty clean just 112000 km. I got out the whole 5th gear set and everything was pretty good the synchros, the gear the fork, everything, so I change the oil again a now put some adittive the car was good for almost a day and half the I make a small roadtrip and the 5th grinding came back but now I notice that it grinds more when is warm.Arial