Oil does not go opaque.
The reason I went to airspaced has to do with the availbility of glasses to achieve zero color. In a triplet oil spaced, you can do this with fluorite in the center because two mating glasses exist that will cancel the color error and the spherical. With ED glass, you cannot cancel both the color and the spherical at the same time. Therefore you either have to use an airspace or you have to use an aspheric surface if you want to do oil spacing, or you have to allow some color error in the blue-violet end of the spectrum. An airspaced ED lens will achieve the same or better color correction than an oil spaced fluorite lens.