Hi Don,
The reason that the yield is declining is because the quality of the crude entering the refineries is declining. Crude outside the 30 to 45 API range require more energy to break down and put together the molecules needed to produce fuels. That requires that the refinery uses more of its own oil to produce the energy. Conventional crude already has an abundance of the C7+ molecules needed to make fuels. For lighter and heavier crude they have to be manufactured from the hydrocarbons present.