I have used Castrol 20w50 in all my vehicles, with the exception of my 97 Vette which called for Mobil 1 5w30. Now I have an 04 Silverado with the 6.0 HO motor, and it dosen't specify synthetic, just 5w30. Any thoughts on if I should go with Mobil 1 or Castrol? I live in a hot climate and have never used anything other than the 20w50 or Mobil 1.