I'm new to this site, but have been reading for the past few weeks. I've been mainly using Motorcraft 5w20 synthetic blend in my 04 Ford Freestar. I've been changing at 4k miles. I've also been using Motorcraft filter. Is Motorcraft 5w20 really that great at $2/qt? Would I benefit from going to Castrol Syntec at $4/qt? I've used Conklin Convoy for years in previous vehicles, because my father is a dealer/salesman. However, his cost is $6/qt when you figure in freight, and it's not readily available. Thus, I have not used it for about 3 years. I just bought a Neon -- which also recommends 5w20. I'm thinking of trying Royal Purple in it. I wish to stick with something available in local stores. Please share your opinions: Is it worth going to a Full Synthetic at twice the cost vs. Motorocraft 5w20?