Needing input on what oil to use. We have an 1987 3607 with Merc gas engines with 340 hp which have about 1600 hours on them. We have only had this boat for a year. The previous owner had the boat for 7 years and in his last year he used a high mileage Mobile one auto oil for the yearly oil change. I used a diesel and gas marine grade oil, 15- 30 weight I believe . I have thought for years that using a synthetic oil in older engines could lead to oil leaks.What would you recommend we use? Compression check when we bought the boat showed 140-150 on all cylinders on both engines. What should we do for oil this season?