Oil vs Air Spaced Lens Elements

Can anyone tell me the pro's and con's of Oil spaced over Air Spaced lens elements. Also does the oil degrade and become opaque over time therefore requiring more attention to servicing compared to air spaced elements. Why did Astro-Physics change from oil spaced lens elements on the 155 to air spacing on the 160.

I am looking at purchasing a 5" refractor and two options are the TOA-130 and the TEC-140.

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.

Roland Christen