10-20 or 10-30 is a good weight
You need to use a Paraffin base viscosity oil like the Motorcraft YN-9........
It is important to have the right types of fluids in a car for it to run smoothly. A Grand Marquis will use engine oil 5w-20 or 5w-30.
( 5 quarts ) with engine oil filter change ( according to the 1996 Mercury Grand Marquis Owner Guide )
5w-30 is what I use in my 2000 grand marquis, in 2001 ford started saying 5w-20.
The 2006 Mercury Grand Marquis owners manual shows : The use of synthetic or synthetic blend motor oil is not mandatory - the oil just has to meet Ford specifications
According to the 2001 Mercury Grand Marquis Owner Guide ( 5W-20 ) ( with engine oil filter change , 5.0 U.S. quarts / 4.7 litres of 5W-20 engine oil )
According to the 2000 Mercury Grand Marquis Owner Guide ( 5W-30 ) ( with engine oil filter change , 5.0 U.S. quarts / 4.7 litres of 5W-30 engine oil )
According to the 2002 Mercury Grand Marquis Owner Guide : With engine oil filter change : The 4.6 liter V8 engine takes ( 5.0 U.S. quarts of 5W-20 )
As long as the engine is ok you can use: 10W30 10W40 I recommend using Vavoline.
For a 2008 Mercury Grand Marquis, it's recommended to use 5W-20 motor oil, as specified in the owner’s manual. You can choose either conventional or synthetic oil based on your preference and driving conditions. Always check for oil that meets the API (American Petroleum Institute) certification standards for optimal performance.
Use a 1999 grand marquis gs key!!
According to the 1998 Mercury Grand Marquis Owner Guide : With engine oil filter change ( Motorcraft FL-820-S ) The 4.6 liter V8 engine takes ( 5.0 U.S. quarts / 4.7 litres of 5W-30 )