I've got a 98 Taurus with the vulcan engine. I bought the car with 33K on it and used nothing but Quaker State 5W30. The manual called for 5w30 and now the car has 118K. No leaks, drips, or errors. The car runs fine and I still get 18-21 around town and 28-30 on the highway. It runs great. I guess all I am saying is that I usually follow the manual for the proper viscosity oil to use and have never had a mechanical problem (engine wise) with any car I have had.