The truth is that the fluids that manufacturer's "recommend" for use in their vehicles are fluids that they have actually tested in their vehicles and have been satisfied with the results.
Fluids that don't fall within the "recommended" catagory more often than not are fluids that were never tested, so the manufacturer has gathered no empirical evidence to their suitability or unsuitability.
Maybe so, Some Mook, but . . . I'm intrigued by those who cling to SOME admonitions in the manual as gospel
, or dogma
, fearing warranty voiding, and maybe worse, from failure to follow obediently the factory guidance. In this case, we give the "W" viscosity a pass.
Then, folklore and mythology play a role; although I'm unaware of any Kawasaki caution (or, kaution), some run, screaming, from the thought of "friction modifying" or "energy conserving" lubricants . . .
Within recommended viscosities and API service codes, I think there's little to fear--ZINC compounds, abundantly present in premium oils (e.g., Mobil 1), are "friction modifiers," for goodness' sake . . .
(I've already confessed to using 5W-40 Rotella synthetic, myself!)