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 1997 Mercury Grand Marquis Owner Guide : ( 5W-30 ) is the PREFERRED oil
I assume it would be ( 5W-30 ) like my 1991 5.0 liter V8 Ford Mustang had from the factory
According to the 1996 Mercury Grand Marquis Owner Guide : ( 5W-30 is the PREFERRED engine oil weight )
According to the 1996 Mercury Grand Marquis owners manual ( 5W-30 engine oil ) is the PREFERRED oil weight
Disc, front and rear.
For a 1999 Mercury Grand Marquis : ( the engine air filter is Motorcraft FA-1668 )
According to the 1999 Mercury Grand Marquis Owner Guide ( and one of Ford's websites ) ( Motorcraft FL-820-S , engine oil filter )
r-12 unless its been retrofitted
Ford calls for Mercon ATF.
10-20 or 10-30 is a good weight
It takes R-134a. Good luck.