It's a long story behind 5-20. This is the short version.

Being in the business for many years, I was told from someone that knows what going on, that the reason some companies went to 5-20 is show better mileage in the cars and trucks by percentage. Why? Gov't ratings. The Gov't wanted to see better mileage out of vehicles so going with lower viscositys, auto manufactures could achieve it.

That said, use 10-30 and don't sweat it. I do and have since 04, the first car I had that recommended it.