Is 20W-50 too heavy for my 340?

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.