It's in your owner's manual. If you don't have the paper copy anymore, you can download a copy from www.fleet.ford.com
under the heading of MAINTENANCE.
If your original owner's manual is missing, download a soft copy from www.fleet.ford.com
under the PARTS AND SERVICE tab.
1995 Mustang GT Convertible, only 68K miles.
2000 Ford Contour Daily drive.
2001 F250 SD, 6.8L, 4 X4.
2007 Mercury Mariner AWD - Wifey's daily driver