Space shuttle has the computing power of a HP calculator. Its also has a lot of parts replaced every flight. It was never a cost effective material launcher, And thats why we are moving away from the shuttle and back to traditional designs. Lower cost, easier maintainance.
20 weight is way light, unless your running some super tight tolerances. Some auto mfgs recommended 20 because it increases fuel economy, even if its not the best thing for the motor.
Look at what subaru recommends for the same motor in japan. Its somehow thicker than the oem spec oil here.