Some gearboxes are a bit sensitive to too much pressure on the detent plunger spring, you can check easily by undoing the plunger cap G 37 a turn or so and see if this is better. The issue with the 5 speed box also is that the gears are harder to engage because the "Dogs" are much larger and fewer of them to a standard 4 speed box, so the shafts have to be rotated much more to engage and shift through the gears. Sometimes with regards to the detent spring I close it up fully in the vice and this shortens it by about a mm or so. Another trick is to change the tip shape of the plunger to a slightly more ball shape rather than such a point, this allows it to move easier as the contact area is a bit less. On some boxes with changes to the 5 speed the camplate spindle pushes down on the camplate making it hard to turn, try cracking it free a half turn and try...............It can be just the bottom of the spindle, or the tinny lip a half inch up from there that just might need a whisker removed if this is the cause. I use 80W/90 gear oil in all the bikes I do, except the racer which I used 40 weight Castor oil. Cheers..................Greg.