What viscosity of oil should be used in a 94 Grand Marquis?

The manufacturer calls for 5W-30. The specifications pages of glove compartment manual will tell you the same thing.

the person 3520 is crazy you can't use 15/40 in a car. I believe Ford is saying that you should use 5w 20w in there cars I would research that not take opinions from here i take my 1998 Continental to the dealer for oil changes and i have no idea what they use but its still running strong with almost 150,000 miles.

Yep. If its not on a sticker or plackard on your oil cap or under the hood somewhere then you can always look up the engine code online and see what oil it cross ref. you to. Also you can look up an old owners manual online or if you have one in your car still iy will definatly be in there.

Good Luck!

what ever the decal under the hood says, it may be on the oil cap