In the past I used Incandescent Oil mostly in Shroud, on my borked but still pretty good GS bow -- holy/shock/holy. If I looked at statistics on that bow, it would show how many minutes of Incandescence were left.
Early this Mabar I put Festival Solar onto my Caustic bow of Acid Torrent, then tested whether Incandescent Oil stacks with it. Yes it does stack: every hit showed two separate Light damage numbers and time left of Incandescence was displayed. So far so good.
Yesterday I put Incandescent Oil onto my triple-positive bow (the borked one deconstructed months ago). The display DID NOT show Incandescence at all, and I think (not entirely sure) there were no Light damage numbers. Has anyone else tried it?