Actually, heat transfer by conduction, convection, and radiation are entirely different processes.
In the case of preheating an engine by forced air (external combustion heater), one is initially using the convection process for heat transfer, then relying upon the conductive properties of the metal.
In a given medium, heat conductivity relates to the thermal resistance of the material. The second law of thermodynamics provides that heat transfer is from the point of highest temperature to the lowest. One will heat the engine the most thoroughly and the fastest by heating from beneath, rather than above the cylinders, and allowing convection and conduction to transfer the heat upward, rather than down.
Heat in oil rises, heated air rises, heat in the engine rises, and yes, when conducting heat through the engine, including the application of heat at the sump, heat rises. The lower portions of the engine will be the first to cool, and should be heated first. After shutdown, the lower portions of the engine cool first, or fastest. Even in operation, the upper portion of the cylinder and upper spark plug tend to receive the greater heat, which is part of the reason that airflow through the cowling is directed at the upper half of each cylinder in a horizontally opposed installation, with airflow generally being aft and downward from there, in a pressure cowl installation.