I have had this problem. I have a 99 Cougar V6 2.5 liter. Please check your owners manual before you do anything but your car doesn't need synthetic oil. You should be using standard SAE oil. If you have a high mileage engine use additives or high mileage oil... Not synthetic. Also check oil weight because if I am not mistaken you should be using at least 5w30. You can use 5w40 for high mileage protection but it will affect your gas mileage. The thinner oil gets even thinner when warmed....thus less protection more friction. You should be idling between 8 and 10 grand, a little below the 1 or at the one on the RPM gauge. Check to see if your idle is correct. If not have it adjusted by a tech.. Don't touch idle screw! The true idle is controlled by the computer and if you adjust the screw your cars engine check light will come on and car will run poorly because the computer is confused. Last but not least is the possibility your crank bearings are going bad. Hope that's not the case. So do another oil change, use correct oil
and filter. If this helps good. If not.... Check idle. If that ain't it.... It's your crank bearings. Good luck.