Before I change my transmission oil (current oil is probably too thick), I need to be sure 100% that this is the cause of the difficult gear changes. I jacked up the left front wheel. After starting the car in neutral this wheel started rotating very fast! That should not happen in neutral, right? When depressing the clutch pedal (in all gears) the wheel stops rotating. So I guess the clutch is ok, comes free. When changing gears and going back to neutral I release the clutch pedal. The wheel starts rotating a (little) bit now and then and sometimes it doesn't. So as the car warms up this issue seems to become less. Is there any other explanation possible for the gearbox to explain this behaviour, or does this point definitely to the oil being to thick in the box?

Perfectly normal for the wheels to rotate when they are jacked in the air. You can stop them easily enough by hand or foot brake. High viscosity of the fluid (when cold) would mean this effect is more obvious, but it can happen even when the fluid is warm.

And also applies to a manual box :thumbs_up:

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