I have a 97.5 Regal GS with the 3.8Litre SuperCharged engine. Manual calls for 10w30. It usually gets some form of Synthetic 10w30 oil. Engine runs great but now has 104k miles on it. Read great reviews on Mobil's Delvac so I decided to try it. Mixed in 4 quarts of the 15w40 Delvac with one quart of Mobil's 5w30 synthetic. Hoping for 4-5k interval along with good cleaning & "superior" lubrication this summer. Good or bad idea on what I did?