My 2007 Corolla had an original spec of 5W30 when I bought the car… A couple of years later I get a bulletin that says 5W20 regular or 0W20 synthetic is OK. Apparently Toyota had enough faith in those light oils to allow them since they were also specified in their NEW cars. Also, their dealers wanted to only stock 5W20 and 0W20 synthetic in their bulk tanks. When I ask for 5W30 in the summer they balk and do it reluctanty.
The Prius was designed to extract every fraction of economy out of the gas, and light oils made it a little more efficient.
However, the engine is basically a 1.5 liter Yaris unit. There may have been may complaints about oil usage with 20 weight, and the heavier 5W30 is perfectly good for a Prius engine, since that was the ORIGINAL spec when used in a Yaris.
So much for the EPA driven politics of fuel economy.