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.
5w-30 is what I use in my 2000 grand marquis, in 2001 ford started saying 5w-20.
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 )
( 5 quarts ) with engine oil filter change ( according to the 1996 Mercury Grand Marquis Owner Guide )
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 2003 Mercury Grand Marquis Owner Guide : ( " regular " unleaded gasoline , 87 octane )
10-20 or 10-30 is a good weight
You need to use a Paraffin base viscosity oil like the Motorcraft YN-9........
Was built for R12, should be converted to R134a
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.
Use a 1999 grand marquis gs key!!