What kind of fork oil should I use for a 2018 Ninja 400?

SOURCE: How much fork oil in a 2002 KTM 400 SX

White power allow a 110mm air gap with springs out and fork compressed as a general guide ,adjust from there according to prefrance . Thats probably,from memory about 600ml or so.If you overfill the leg ,cut the tip off the fork oil bottle , mark out on a clear piece of fuel hose 110 mm from one end of hose with a permanent marker, attach other end to the tip of the now empty fork oil bottle nipple, squeeze bottle,insert hose down to 110mm mark,take hand pressure off bottle and WWHHAAALLLAAA!!!!!!!!. Magically the excess oil is sucked back up into bottle.....Don't you just love the forces of nature. : ] ride on DB ......Hope this gets you out of trouble ....Flabbycat

SOURCE: FORK OIL CAPACITY FOR 87 NINJA 1000R

ex500 models others will be similar it takes 352ml of oil
per fork. 15wt oil is what is manufacures speck for fork oil

SOURCE: Fork Oil Question

Gays????
No fork oil should destroy seals. I wouldn't go crazy on brand, as this isn't a full blown motocross bike. Use what you can get easily. Changing it is the important part, as many people never do.

SOURCE: 1988 kawasaki ninja 600.weight of oil and how much

Depends how stiif you want your fork to be, it goes from SAE 5 (very light) to SAE 30 ( very hard).
For engine oil, normally you can use mineral oil, if you have a bit more money best to use semi-synthetics. Cheers

SOURCE: Volume of fork oil needed per fork for a 1986 VF 400 nc 13

Well, when you take off the fork make sure you empty it first before anything else then make sure that the fork pressure spring is released before changing the seals. The fork oil amount should be no more than 200 cc or no less than 190 cc