I would get a gallon of wax and grease remover & with a clean paint brush and scrub brush go over it vigorously repeatedly with enough of it to where it is dripping a big mess under the eng. then finish up with several cans of brake kleen, drowning it & it'll get into the pores/nooks and crannies. I would not use any rags. They were referring to painting a flat surface but the paint guys at Sears once told me that you use solvent (of an appropriate type of course) to raise the oils/debris then you must wipe it off when wet otherwise if it dries the oil just goes back down to the surface. I dont think the oil will evap no matter how much it is diluted but instead it must be flushed off of the eng.