When you applied oil, did you wipe or brush it on? If there is staining or debris in the pores of the plastic, using an AP brush to apply oil and work it into the pores may dissolve and break up the staining. You may want to try cleaning it first with a mild detergent like Dawn or purple colored Simple Green.
If that doesn't work, hit it with a mild polishing compound like Flitz, white Scotch Brite or 0000 steel wool w/ oil. It may be the finish was damaged and is reflecting light differently enough to account for the color difference. If that's the case, no amount of oil will fix it since a uniform finish needs to be applied to the entire piece. A soft wire wheel (like a carding wheel) may work too but use LIGHT pressure. Keep in mind that any polishing (especially a with a carding wheel) can make a surface shiny if you go hard. If it gets too shiny, you can always dull it again with Scotch Brite. Avoid a standard wire wheel as it will eat through plastic.