I have a bottle of Labelle #108 that, at this point, is probably 10+ years old. It seems to me it has turned color in that time (its now the color of weak coffee). Labelle sez its a 'light' oil, suitable for small engines, like HO and smaller. A few years ago, I bought a bottle of XL light at my local hobby shop, in part because I was concerned about the Labelle discoloring and whether that was an indication of some polymerization of the oil. I think that XL light has discolored also - sorta tea colored (I don't have a 'before and after' picture to compare - just an aging memory). One thing I did notice just holding the two bottles up is that the Labelle is definitely more 'viscose' - just based on the behavior of the air bubble at the top of each bottle. Again, I don't have a 'new to old' comparison to go by. The Labelle has been consigned to my woodworking shop, where I use it to lube bearings on router bits, etc.
Recently I inherited a large bottle of sewing machine oil from my wife, who now has (an expensive) sewing machine that doesn't require lubrication (about which I am skeptical...). That oil is as colorless as water and its supposed to be 'non-staining'. By eye, it seems to have the same low viscosity as the XL light mentioned above. My guess is that this sewing machine oil is at least the same age as the XL light. So I wonder how well some of these oils age - and whether a change in color is simply a change in color or might indicate that there is some polymerization going on.
BTW, there is an advantage of your wife/SO owning an expensive sewing machine (or loom, or ...) -- I have some catching up to do 
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