Sorry about the engine error - my first 4 cylinder car. Anyways, according to the link below, they [tree loving hippies] wanted to increase fuel economy and reduce emissions nationwide, so the govt required 5W-20 oil on newer cars. However, the difference is neglible so 5W-30 is fine.

According to someone I went to school with - his work was in tribology, or friction at the molecular level - Honda was finally able to cost-effectively machine parts with tighter tolerances that benefit from lower viscosity oils.

So, the truth is probably somewhere in between.