Surprise milege

here's a surprise. I have been getting about 43MPG in city. My Prius has 790 miles on it. We went on a trip that was about 200 miles- 90% of it was highway- cruise set @ 71 MPH. I noted when we got home we averaged a solid 48.5 MPG on the h-way.

I liked that as I didn't expect that much. Does anyone who is using full synthetic oil know for sure if their MPG has increased? Many times syn. oil will help milege as it is very slippery and the engine parts have less friction.