Manufacturers used to use trick oil for the official MPG and HP tests. They would run lighter oil in both cases to free up a little HP and reduce the gas consumption.
Now, the federal government mandates that whatever oil the MFG uses in its testing must also be the official recommendation in the manual/oil plug. As such, they can no longer build an engine for 10W30 and test it using 5W30.
That, combined with newer materials and tighter tolerances mean that pretty much all newer cars will require 5W30 as opposed to 10W30 as they used to.
