You probably have 10W-40 in there now. I don't think Castrol makes a 20W-40. That would account for your additional top end noise. Switch to a 20W-50 of some sort and I think you'll cure the excess noise problem. Any brand is fine.
Due to the crude oil filtering system on these, changing the oil often is the best practice. Most of us go about 1000 miles between changes. Expensive synthetic or semi-synth is really of little benefit to these. You're dumping it before you can utilize the high change intervals it provides. As I said, any cheap dino 20W-50 will do. Personally, I use the Walmart brand.