I used to use Lucas Oil Stabilizer on my old high mileage 4.0 back in the day, and it did make it a lot smoother and quieter. I used to put in a quart of Lucas and 4 quarts of oil at an oil change, for a total of 5 quarts. I think the bottle says you can even go up to 50/50 mix if you need to, but I would imagine that would be really viscous and thick.
Right now you could add some Lucas to your crankcase and just keep an eye on the oil level so it doesn't go above Max on the dipstick. If you're worried about overfilling you could drain some oil first, or just do an oil change.