The lower portion of the firebox now on #19 is not the same as it was when she burned wood or coal. Reason being, there is no provision for ash collection, which there doesn't need to be.
Concerning brickwork. The brick goes up only part way on the sides covering about 20% up the side, rear flue and firedoor sheets. Once those bricks are hot, they remain hot for quite some time. At night when the fire is out, they glow for quite awhile. The upper portion of the sheets are exposed as is the crown sheet, so the heat from flames strike them directly, then the exhaust gases go through the exposed flues extracting more heat. Btw, a smokey fire causes three issues. 1) Smoke is unburned fuel and therefore does not heat the flues as well. 2)The smoke contains a lot of carbon which deposits on the flues causing a need for sanding. 3)It is a waste of energy. Fireman are judged by how they keep the water and steam up, and the cleanliness of the stack exhaust. A good Fireman runs a clean stack and keeps up the water lever and steam presssure. At SVRy we try to minimize flue sanding because some of the sand stays in the fire box and causes the brickwork to shift.
Back to the bricks. It seems to me that there is less area for the flames to heat and therefore maybe more oil is required to make steam than the equivalent amount of Btu from coal. I don't know for sure the exact reason for the brickwork, and will have to read up on that issue.
As for the oily mess that Greg saw in Spain. Cleanliness is a function of workers and provisions to keep facilities clean. (Ultimately it is a management issue.) It doesn't have to be messy. And it depends on your definition of messy. I find coal to be pervasive and resides everywhere in a coal burning loco. Oil in a cab can be wiped up easily. I think it depends on that to which you are accustomed.
dan