What is your total weight with gear on?
In section 13-6, the front fork suspension oil for the N400 is the Kawasaki SS-8. Which is a 10 W oil.
You could try a heavier 15 W oil.
Standard fork oil level with the fork fully compressed without the fork spring and spacer is about 88 +/-2 mm. Measured below from the top of the inner tube.
Standard front fork spring is a dual rate spring also called a progressive spring with a spring rate of 14-21 Nm.
The fork oil swap is a band-aid fix. It may help, but, you'll want the right spring rate for your rider weight.