My owner's manual recommends SAE 10W-40 and states that this viscosity is rated between 14 and 104 degrees F. I changed my oil last weekend and used Valvoline SAE 10W-40 motor oil (it was not labeled as being specific to motorcycles). I have only had 3 rides since the change...maybe 60 total miles...but I have not had a problem at all.
So now I'm curious Blue...why is everything you have listed a 15W instead of 10W? Is that generally what is recommended by the community, for best overall protection? I just find it odd that I may not be providing the best protection by following my owner's manual.
Not to get into a big oil debate...OP said, "i can't find no other oil, just valvoline.."
. I just gave him some very suitable alternatives. Although the owner's manual recommends 10w-40 it also says,
Recommended Engine Oil
Type: API SE, SF or SG
API SH, SJ or SL with JASO MA
Viscosity: SAE 10W - 40
Capacity: 1.3 L (1.4 US qt) (When filter is not removed)
1.6 L (1.7 US qt) (When filter is removed)
○Although 10W-40 engine oil is the recommended oil
for most conditions, the oil viscosity may need to be
changed to accommodate atmospheric conditions in
your riding area.
What is also listed in a chart is 10w-30,10w-50, 20w-40 & 20w-50
What it does not list is 15w, which falls in between 10w & 20w
The 15w oil I listed (although auto/truck oil) meets the type requirement recommended by Kawasaki and recommended by many in the motorcycle community
You will be fine with 10w-40 if it is in the temp range you ride in. As for me I ride longer distances at higher speeds more than the average rider. I'm also a little heavier too, so I use 20w-50 semi synth and change it frequently. Why? because the book says I can. Besides, it makes the bike ride smoother. Keep in mind the Ninja 250 is a high revving bike and any oil you put in it will breakdown. How fast, depends on your driving conditions.
Everybody has their own choice of what they use. Kawasaki gave us very good options, that they know the bike can handle.
Like Spooph said, "some oil is better than no oil"