Low gas mileage

About 2 years ago I bought a 2000 Ford Focus ZTS w/ the 2.0 Zetech double over head cam engine. The car had only 11,000 miles on it as it was owned by an elderly friend. I’m disappointed with the gas mileage. It averages around 25 mpg and I drive mostly highway (~20k per year). I previously owned a 1994 and 1998 Mercury Tracer both with the 2.0 engine but not double overhead cam. My driving habits nor my route have not changed and my tire pressure is fine. I have friends with newer models of the Focus who are averaging in the 30+ mpg range. What is wrong with my mileage? And also, any harm/benefit in going with oil that is 5W/20? My owners manual states 5W/30, but the newer Focus is recommending 5W/20.