To change the motor oil of a Kawasaki Ninja 400, start by lifting the bike using a suitable stand to ensure a stable and safe position. Under the engine, locate the oil drain plug, usually located at the bottom of the engine block. Place a collection container under the plug and loosen the bolt using an appropriate wrench, allowing the used oil to drain completely into the container. Once the oil has been completely drained, reattach the drain plug and tighten the bolt with the same wrench, making sure it is securely fastened to prevent leaks. Then, locate the oil fill point, which is usually located on the top of the engine or near the oil fill cap. Remove the fill cap and, using a funnel, slowly pour the new motor oil into the cavity, following the capacity and type of oil specifications indicated in the user manual. After adding the oil, replace the fill cap and make sure it is securely screwed on. Start the engine and let it run for a few minutes to allow the oil to circulate inside the engine and check for any leaks. Finally, turn off the engine and check the oil level to ensure it is correct. If necessary, add more oil until reaching the appropriate level. The oil change is now complete and your Kawasaki Ninja 400 should run smoothly.