HI!
I'm working in a refinary and i've a couple of cases that paid my attention. I took an oil sample of ISO VG 46 turbine oil and it was yellow tending to orange brilliant and clear color. But in a couple of hours it got dark. The lab analysis indicates a color 7 (on the scale top 8), but the TAN number was normal (0,09 mg KOH/gr), and the rest of properties were ok. I took the same sample later in a bottle covered, and stored it for a day away from light. the result was that after 24 hours the oil remained bright and clear, but then it began turning dark and now it looks black. Does anybody know this effect? oxidation with light?

Alvaro,
You might have contaminated this oil sample in the process of taking it. I say this, because I’ve heard the exposure to UV light is a sequence of the Mobil's comprehensive compatibility testing between oils. If any of the tested mixtures exhibit similar to what you have seen, it would be viewed as a sign of incompatibility, or one may say - contamination. I don’t think anyone can give you answer with any certainty regarding darkening of oil without knowing chemistry of its additives, and extensive testing by using forensic type of testing (unconventional methods). However, I see your observation as a non-issue for majority of end users, because their oil will never see light of day, especially when almost everyone tries to "plug" breather openings, the only areas UV light might reach oil, with desiccant breathers in order to keep contaminants (hard particles and moisture) from reaching oil in sumps.