I purchased a 2005 Ford Mustang GT with 96000 mi. 5 sp.man. trans. from a dealer with a decent service record and car fax report. The vehicle is in storage for the winter and what oil for it's coming out party this spring. Factory calls for 5-20 conventional. What would you use.
Use oil that meets the manufacturer's specifications.
Factory calls for 5-20 conventional
I've never seen a manual specifically call for conventional oil. Usually it gives a minimum API specification.
Factory calls for 5-20 conventional.
Then, use 5W-20. Mechanical engineers designed your car to specifically use 5W-20, so, use it. Long gone are the days when you could use a range of oils depending on ambient temperature when it's cold outside. I have a '79 Pontiac that uses a range of different oils depending on the degree of cold weather. Piston rings are not nearly as tight as they used to be in order to get better gas mileage, among hosts of other changes. Modern cars use much thinner oil than old cars because of this.
