15W-40 is a lot thicker than 0W-40, 5W-40, and 10W-40, even when hot.
While the SAE KV @ 100 C range is the same for above oils, the SAE HTHSV min for 15W-40 is more (3.7 cP vs. 3.5 cP). (KV = kinematic [low-shear] viscosity, HTHSV = high-temperature, high-shear viscosity.)
In practice, the difference is even more. For example for Mobil 1 0W-40 and Mobil Delvac Super 1300 15W-40, they are 3.8 cP and 4.3 cP, respectively. This is a huge difference in HTHSV, as the wear protection and fuel economy (internal oil friction) in a warmed-up engine are solely determined by HTHSV and even a small difference translates into a big difference in wear protection and fuel economy (internal oil friction).
HTHSV refers to high-shear, high-temperature viscosity. Shear means closely sliding parts. Between fast shearing parts, where the oil film is only microns thick, the temperature of the oil suddenly rises to much higher temperatures than the bulk-oil temperature elsewhere. Oil exiting the thinnest section of the oil film suddenly cools down to the bulk-oil temperature. Also, the fast-shearing viscosity (HTHSV) of the oil is smaller than its slow-shearing viscosity (kinematic viscosity [KV]) at the same temperature, as the viscosity-index improvers that increase the viscosity are temporarily sheared during high shear.
All this said, I also looked at the oil recommendations by Nissan for this engine, and they are all over the place depending on the country where the car is sold. The two below seem most standard among miscellaneous countries:
My recommendation:
Conventional 5W-30: Do not use, as it will shear to 5W-20 or near-5W-20 viscosity (both KV and HTHSV) in less than 1000 kiometers.
Fully synthetic 5W-30: Excellent choice if oil consumption is not high. If oil consumption is high, it won't be an economical choice. This would be my preference for fuel economy and adequate protection.
Conventional 10W-30: Excellent choice, with only slightly worse fuel economy than fully synthetic 5W-30. Also slightly shorter oil-change interval than with fully synthetic 5W-30.
Conventional 10W-40: Good choice but oil pressure will be more (more strain on the oil pump) and oil temperature and bearing and ring temperatures will be higher. Also, fuel economy will be less. Added wear protection in certain driving conditions (high-temperature, high-speed, high-load, or some combination).
Fully synthetic 0W-40: Same as conventional 10W-40 with better cold-engine fuel economy, however much more expensive.
HDEO 5W-40: Avoid as HDEOs contain additives that are neither needed nor entirely suitable for your gasoline engine. However, it will work. Expensive as it's a fully synthetic grade.
HDEO 15W-40: Practically the thickest oil you can get, just about as thick as 20W-50 as far as HTHSV -- the critical viscosity -- is concerned. Avoid it, as the fuel economy would be the least and the engine friction, oil pressure, etc. will be the highest, despite excellent wear protection in the most-demanding driving conditions (as described under 10W-40), with the oil film being the thickest. Same reservations as 5W-40 HDEO apply. However, it will work and it's a good choice from an economical point if your engine is consuming a lot of oil, as the oil consumption with 15W-40 will be the least and it's also a cheap to get oil.
