Depends upon the application.
15w-40 would be ok in most vehicles, although some small amount of fuel efficiency may be lost.
Engines with hydrualically actuated VVT would possibly suffer though, as they are often spec'd for a certain grade to perform as designed; higher vis may retard or otherwise affect the VVT.
Also depends upon why you are considering it in the first place. If you're doing it to reduce your lube inventory, I guess that makes sense. But if you're doing it because you believe that somehow the wear protection will be "better" than a good quality dino 10w30, you're likely mistaken ...