Well I guess there are really two parts to this dilema:

1- Why does Honda tell everyone to use 5W-20? Any time I've seen this in the past it has been to boost EPA ratings (lower viscosity oil tends to generate better fuel economy because it takes the internals less effort to push the oil out of the way). But I've always heard that 5-W20 also tends to make engines wear out a little quicker because it doesn't do as good of a job at coating and protecting the motor.

2- The dealership told my buddy that synthetic oil (Mobil 1 in particular) was bad for 4-cyl Honda motors and that he should only use Honda's regular oil. Is there any real reasoning behind that at all or was he just trying to make money and keep him routinely coming in for service?