"The 20 in 0W-20 indicates the viscosity characteristic of the oil when the oil is at high temperature. An oil with a higher viscosity (one with a higher value) may be better suited if the vehicle is operated at high speeds, or under extreme load conditions."
Such an instruction in the manual might indicate that a heavier viscosity oil won't screw up the VVT, clog those "tiny" oil passages, and that the engine wasn't "designed" for using only a thinner oil.
It all depends on what they mean by higher speeds? . They run faster RPMs accelerating in town than they do at 60 on the interstate.