Not just can, but should. Ford recomends full synthetic for both the V6 Duratec and the I4 Zetec.
Synthetic oil can boost your horse power. Also fuel additives.
5W30 full synthetic.
5w 30
I like to use Valvoline synthetic and have had good luck with it.
at the bottom of the oil pan under the car.
According to the 2001 Mercury Cougar Owner Guide : For the 2.0 litre four cylinder engine : ( with engine oil filter change , 4.25 litres / 4.5 U.S. quarts of 5W-20 engine oil )
Use Super Premium SAE 5W-20
On a 1991 Mercury Cougar : The oil pump is INSIDE the oil pan ( the oil pan is bolted to the bottom of the engine )
The oil drain plug is located under the front end of the car, on the rear side of the oil pan.
The correct oil weight is now 5W20; it used to be 5W30, but Ford has recently allowed your engine to use 5W20 as an energy saving oil. If you don't use synthetic oil, stick with the 5W30; non-synthetic 5W20 will burn out your engine faster than non-synthetic 5W30.
5w30
5w 20 ======================================================== The above answer of 5W-20 is correct if it is a 2001 or 2002 model year according to the Mercury Cougar Owner Guides ( 1996 , 1997 , 1999 , 2000 guides show 5W-30 from the factory )