My owners manual recommends 10W30, 5W30, and 15W40 for use in my Turbo motor. I have always ran 10W30 but with the warmer tempertures I'd like to run a thinner oil for better mileage. I don't want any problems with running a TOO thin oil. Next change will be due somewhere around the mid to end of April so don't need to worry about cold temps. Would it be better to run the 15W40 or 5W30 during the summer.