shouldn't be. as most 0W-40's go, M1 starts out closer to like a....35 weight, really. shears down initially from a 40 to a high 30 range but then settles and holds there very well with its strong High Temp High Shear rating. its cheap, easy to find, and has a very effective additive package for long drain intervals. hard to go wrong.
i will likely be doing exactly what you describe. my other vehicle runs 0W-40's and living in a hot SE climate, along with towing, occassional heavy loading, off road use, and high speed cruising (all fall under severe use in the manual) makes it seem like it would be a perfectly fine fit for our 4runner, and i could just keep 1 kind of oil on the shelf anyway.