have 2005 mazda tributebuilt by ford) 3.0ltr v-6, have always ran synthetic (valvolne or mobil1)5w/30. the oil filler cap says 5w/20, but based on 1st mob info, etc, always went with the 5w/30. Never had any issues. does it make sense now for any reason, to go to the lighter viscosity, 5w/20 now, at this time, with 135,000 miles, or stay the course, and does the 5w/30 provide more protection , in general, than a 5w/20. thanks