I use Mobil-1 in all of my vehicles. IMO, it is one of the best motor oils on the market.
That said, it is a motor oil, not a gun oil. It has additives to stabilize its viscosity, remove varnish buildup, prevent foaming, prevent acid formation, provide extreme pressure lubricity, and a host of other things that gasoline engines need but, none of these things are needed in a gun.
Like the other motor oils, if it is present inside the barrel in a muzzle loading gun it will mix with the black powder fouling and form a hard, difficult to remove fouling. Because of this and the fact that almost none of the additives are needed in a muzzle loader I don't recommend using it.
There are a lot of different gun oils out there made specifically to meet a guns needs so I think using one of them is a better idea.