You don't have a lot of options in 20w50 these days. All the car manufacturers are going to 5w20, 0w20 and WD40 to get that last 1% gas mileage. Out of all the oil choices on the Walmart aisle, for 20w50 it was either Castrol, Pennzoil, or Supertech/some generic brand. My past experience with Castrol in my old car was that it broke down fast and felt like water at 1500 miles. I chose Pennzoil and it held my mildly rod-knocking KA together for several thousand miles of abuse before I switched to Mobil 5w30 and killed it for good. (I thought the rod knock was some sort of valvetrain noise because it was very mild. It wasn't so mild after running the car hard on the lighter oil for a week.)

I'm probably going to compromise and run a 10w40 or 15w40 in my new-used KA. The manual recommends 5w30 but with an older engine and the kind of abuse we put on them, after my recent experience I think I'll be going a bit thicker.