Problem in gearbox or clutch?

Hi,
I gave strange situation.
When car is cold i can shift gears good, but when im driving for ~30min gear change becomes really hard. On reverse, there is grinding noise etc.
Also when i park on slope and leave in gear, car starts to roll. In reverse it stays in place.

Changed oil in gearbox, but it didn't solve anything.

Any suggestions what has gone wrong? Thanks!

Sounds like it has the wrong grade of oil in it. It MUST HAVE gl4 specification oil in it not a gl5 or gl4/5 combination oil. The gearbox is very particular about this.

Oil change was done is car service. So i will ask what type oil they put in.

But there wasn't any noticable difference before or after oil change.

Shouldn’t roll down hill in gear
Is your clutch slipping?

Is this with the handbrake on?

If you are stationary and engage 5th gear and lift the clutch slowly, once at the biting point does the engine stall? If yes then the clutch is fine, if the engine keeps running then the clutch is bad. A slight roll that takes up the backlash in the drive system is normal but shouldn't be more than about 150mm.

Lamberts technique is the best for checking the clutch.
If you have clutch problems you are more likely to notice problems shifting in 1st and reverse because there is more torque going thru it.
From your what you describe, your clutch plate might be breaking up rather than worn. You can check for this by resting your foot on the clutch in neutral with the engine running. A pulsing thru the pedal may indicate a problem.
Before you haul the gearbox out though, have a quick look at the clutch fluid level and around the slave cylinder for leaks. Clutch and gearbox may be fine but because the slave cylinder isnt moving enough it shows the same symptoms as not pressing the clutch down fully.