Ron, a 5 mpg drop in MPG is fairly significant if it happened on a round trip that you have done several times before.
I know that the weight of the oil can have a small effect on fuel economy, along with timing, tire pressure and all the stuff that Bartamos mentioned.
I would probably take a look at the timing first, as that is an easy adjustment, then maybe use a liquid "tuneup" to see if that might clean up the carbs a bit.
