While it dosen't really help on warranty issues, I wonder what the Chilton or Haynes aftermarket manuals says for oil grades. My '95 F150 manual says to use 10w30 or below 60F I can use 5w30, but the Haynes manual for my truck allows all kinds of grades including 15w40. Seems to me the aftermarket manual is basing the recommendations on actual mechanical considerations, whereas the manufacturer bases them on getting the CAFE fines down to a minimum. We all have seen the EPA letter that they must use, promote, and practically ensure the use of 5w20 merely so they can use it for the CAFE tests.