Shouldn't harm anything.

A couple of thoughts:

1.  A lot of variables can affect gas mileage -- including seasonal
changes in both driving style and the formulation of gasoline.  It's a
hard experiment to control properly.

2.  Not all synthetic oils are rated as energy saving -- and some
dinosaur oils are!  Although synthetics are often perceived as being
more slippery, that isn't the only issue.  It has more to do with
viscosity and formulation than with feedstock origins.  It is also
supposed to be interpreted as a fleet average rather than as a
guaranteed result in any particular car.  Finally, it is only one or
two percent at most -- one of those factors that are a big deal
societally but not necessarily apparent, let alone important, to the
individual consumer.

I think you've appraised the situation correctly  Synthetics are
better, especially in hard service -- but you can get a good long
engine lifetime out of a conventional oil if you change it, w/quality
filter, on an appropriate schedule.    

Best of luck,
--Joe