Since I have cases of both out in the garage, I plan on mixing 3-1/2 quarts of Mobil 1 0W-40 with 2 quarts of Mobil 15W-50 to make a "witches brew" of approximately 5W-45 for use in my modified Taurus SHO V6 during the warm Summer weather. The car is only driven from April though October and sees occasional track time on regional road courses. The engine is no where near stock, with a higher compression race prepared stroker short block and full prepared heads.