If you developed a leak after you changed over its because the old oil had gummed up the leak before and you didn't know it. If your going to run Synthetic oil run Amsoil its the best on the marked and you get Extended change intervals with their filters. I run a powerstroke diesel and I change my oil every 30,000 miles and my filters every 15,000 miles. Now at about $110 for a change and two filters. I save alot of money per year versus a change every 5,000. Plus it takes 15 quarts per change.