I was considering Royal Purple 5W-30 for the Mercury and i know Redline has a 0W-20 that is actually the lightest possible 30-weight. Now, i dont feel like paying for Redline to be shiped, and im in a warm climate. Therefore, how heavy is Royal Purple's 20-weights, particularly the 5W-20 which i can buy on the shelf at the store? I feel 0W-40 would be a poor choice for the car.. maybe. (Same with the 5W-30, wally world or the store with the 20. 10W is not entering my thoughts.) Royal Purple's 5W-30 might be good, instead of a 20 or 40. But how good are the 5W-30, 5W-20, and 0W-20? Me thinks this engine needs a gooood cleaning and I would not overlook RP as a boutique oil. Particuarly since the oil in it so far has been so bad and the engine rattles at idle. (Diesel sound.) I also cant change the oil for another month or so, btw.