Megane 3RS Cup 250 - Hard Shifting at high RPM after oil change

Hi everyone,i own a Renault Megane 3 RS 250Cup Stage 2 with the PK4 gearbox with LSD.I'm experiencing problems with gear selection at high RPM ,and I'm trying to determine whether the gearbox oil coul be contributing to the issue.

The symptoms are
-With the engine switched off all gears engage smoothly
-With the engine running at idle gear selection is totally fine
-When driving and shifting at high RPM the gear level becomes very stiff and sometimes the gears wont engage until the RPM drops below approximately 3000 RPM
-If i wait for the RPM to drop before shifting the gear usually engages more easily
-Sometimes if i try to engage a gear immediately if starts to engage but feels like it wants to grind ,if i wait a moment and try again the leveler feels stiff

What has been done so far
-The gearbox oil was changed to ELF NFX 75w as the recommended ELF NFP oil was unavailable-
-Approximately 2.65 liters were initially added ,but we reduced the level to approximately 2.4 litters
-The shifting improved noticeable after removing around 150ml of oil but the problem has not completely disappeared
-The clutch hydraulic system has also been worked on ,including replacement of ghd upper clutch hydraulic cylinder/pump and bleeding(3 times)

Before the oil change i did not have exactly the same shifting behavior,the oil oil was ELF NFP75W-80

My questions are
1.Could ELF NFX 75W with its relatively low viscosity be contributing to these symptoms?
2.Has anyone experienced similar problems with the PK4 gearbox after changing the gearbox oil ?
3.What gearbox oil specification and exact product do you recommend for the PK4 gearbox with LSD ?
Thanks in advance for any advice!