The 20/50 will run fine. It's not what I would run or recommend, but big whoop. I know this much. Every single person (and that's a lot) I've ever seen talk about 20/50 online or in person says they've gotten great service from it. I had some, I tried it and didn't like it. But that doesn't mean you'll have the same reaction, so run it. It certainly won't hurt anything.
Every app. is different. There is no one answer. A all-out race car, I would use a thinner oil, as you say, but I would also use a High volume oil pump, which would also mean a deeper pan.