I have been using Mobil delvac 1300 super 15w40 in my 1997 F250 7.3 since I purchased in new. Now has 270,000 on the clock. Change every 4000-5000 miles. No problems at all! To my surprise. Ford doesn't recommend it. Mobil doesn't recommend it in Turbo diesels. I want to stick to Mobil, but which one? The 2 obvious choices are Mobil 1 15w40 extreme synthetic or Mobil 1 5w40 turbo diesel truck synthetic. I might like the 5w for starting ease in winter. But wouldn't want to swap to something else for summer. Any thoughts?