If it's dripping a whole bunch of oil when it's running, then yes, it's necessary, and no it isn't the lower manifold gasket. There's a stub shaft under the throttle body that has a crispy O-ring around it. It's the part that allows your camshaft to turn your oil pump. If you remove the throttle body and the fuel rail supply and return lines you can pull it right out and replace the O-ring without removing the manifold. I'm assuming you have the 2.8L or the 3.1L, but you weren't specific. If you have the 3.8L and it's really coming from the manifold, then it's probably insignificant seepage.
