[Mustang_Driver]

if i were you i would get a jug of solvent a stiff bristle brush not a wire brush!!! and a bucket and let the manifold soak for a little while and use the brush to remove the CRUD..... but with that much crud this looks more like a lot of Blow-BY past the piston rings but that's another animal altogether..... but like i said soak the manifold in solvent try and clean as much of the crud as possible and re-install after you get it put back together i would get a compression test done ( Just because with that much build up it looks like oil is getting past the piston rings).

But like all mechanics say clean parts are nicer to handle...and gaskets will seal better

Hope i helped