losing oil

My 99 Ranger with a 4.0 seems to be losing oil. I started out with 5W-30 when I bought it a few months ago and it seemed to run through it. Switched to 10W-40. Same thing. Should I switch back to 5W-30 and use Lucas oil with it? I know Lucas is a great product so I'm just wondering if that might fix the problem. The average loss is about 1/2 quart every 200-300 miles. New valve cover gasket. I don't see any visual smoke any time. Any help is greatly appreciated!