Hello Scott,
Rover introduced the composite head gaskets for the Rover V8 with the introduction of the 4.0 and 4.6 litre engines. These engines were fitted with an all new head design, which featured 28cc combustion chambers for specific use with composite gaskets, and 10 bolt head retention.
Cylinder heads made prior to 1994 all feature 36cc combustion chambers and 14 bolt head retention. These heads were designed to be used with pressed metal gaskets.
You can of course use composite gaskets with the 14 bolt heads, but the CR is reduced as the gasket is thicker than the pressed metal ones, for which these heads were designed to be used. As harveyp6 mentioned, using composite head gaskets with 14 bolt heads will also requre the use of a composite inlet manifold gasket.
Composite gaskets provide improved sealing when compared to the pressed metal design. I don't know about the graphite head gaskets Scott, but I would assume that they too are of a fibrous nature like the composite ones. If you go to the local Range Rover spare parts store, they will have the Rover composite gaskets.
Rover are very specific about NOT using any sealant on cylinder head gaskets. This applies equally if using pressed metal or composite gaskets.
Rover also ammended the torque settings applicable to the cylinder head bolts on 14 bolt heads. (Ref....1997 Land Rover Engine overhaul manual covering 3.5, 3.9, 4.0, 4.2, 4.6 litre engines.) Specifically, when using pressed metal gaskets, the outer row of bolts..numbers 11 to 14 the torque for these bolts is reduced to 44 lbs/ft where as for all others the torque is 66 lbs/ft. If using composite gaskets with 14 bolt heads, then these torque settings will also apply.
Ron.