Is Oyster Stout really made using oysters? One of my favorite stouts is Oesterstout by the Schelde Brewery.

The website states:


  During the brewing process, the wort of the beer is pumped across the oyster shells.


Is this what gives it it's distinct taste? In Denmark, another more easily available Oyster stout is Marston's Oyster Stout. Do all Oyster Stouts follow this particular process, or is it more of a sales pitch?
Oyster stout traditionally uses oysters as part of the brewing process, and that is the flavour that differentiates them from other stouts.  While it is traditional to use oysters, some modern breweries use artifical flavours in their oyster stouts, or simply say that they are intended to be eaten with seafood.

You mention Marston's Oyster Stout, which is one example of an oyster stout that doesn't use actual oysters in the brew.  From their website:


  Marston’s Oyster Stout is a dark, creamy, smooth, clean tasting English stout. It doesn’t contain oysters, just called Oyster Stout as this style of ale is a great complement to shell fish dishes.