Stop worrying. This is a small engine. It is geared to operate a what may appear to be higher than anticipated rpms. At 70 MPH you will be around 3,000 rpm's.

What you think of as working hard is actually the opposite. These engines will run all day at 5,000 rpm.

My '92 300E has 242,000 miles of the last 140,000 90% were on the highway at between 70 and 80 mph. Valve train is still original. Highway miles at over 3,000 rpm are not harmful to this engine.

Steve