Hi and welcome to FOG.
A few things can cause notchy gears on the sport, make sure its engine oil in the gearbox, not gearbox oil, I know it sounds silly but thats how Isuzu boxes are
The 20l does tend to have notchy gear change when the engine and gearbox is cold, after its warmed it gets better, also when you take the gear stick out there are little white plastic bushes that can wear over time.
Remember to check the clutch fluid as well and bleed it, there are all the simple fixes
