I did an extensive search here and could not find answers so I am posting new. 2002 Dakota 4.7 with approximately 10,000 miles. I switched to Castrol Synthetic 5w-30. (I know they are the company that started the whole controversy of "what is a Synthetic Oil?"). Also installed a Purolater L3000, a full sized filter instead of the half sized. I added an extra quart for the larger filter size. Questions: Castrol Synthetic 5w-30, Group III or IV/V? Good Choice? How long until the next oil change? Most miles are highway with some in town, I'd guess a 60-40 split. Purolater L3000. Good Choice? bypass? cardboard?