So I am trying to trace down a slow coolant leak on this LS-1 Miata of mine. I am pretty sure it is in the radiator. But to be sure, I am doing the UV coolant dye trick.
For those who do not know, one can get a dye you put in your coolant that glows very brightly in UV light a.k.a. a 'blacklight'. You put some in your radiator, run the engine for awhile, and look for leaks with a UV light. Works great. FYI I bought the dye at the auto parts store, and the UV light at home depot. It is a compact fluorescent type bulb.
I had a serious "oh-****" moment tonight when looking around the engine in the dark with the UV light. I noticed that a small amount of oil residue around the oil filter had a slight glow to it, the same yellowish color as the radiator dye. So, even though I had seen no other evidence of coolant in the oil (milky oil, water bubbles in the oil, rising oil level, etc) I immediately start having horrid thoughts of a blown head gasket. That is, coolant has UV dye, coolant gets in to oil, oil glows.
So I check the dipstick with the UV light, and sure enough, the oil on the dipstick has the same yellowish glow. Damn it...
After hating life for a few minutes, I decide, just for grins, to see if the Rotella-T synth I have in the engine right now glows on its own. So I grab a bottle of the same Rotella-T I had put in the engine at the last oil change. There is about a quart of clean, unused oil left in it. I poured a little in the cap, and sure enough, it glowed in the UV light, the same yellowish color. A small drip of the same new Rotella that I splashed on a non-glowing surface also fluoresced yellow. I then held the capful of the Rotella and the dipstick up next to each other in the UV light, and they had the same glow to them. Same color, same brightness.
Whew... life is OK again.
So, I have no idea if this works with other oils, or what the additive is in the oil that causes this glow, but there it is. Rotella-T synth glows slightly in UV light. It is not nearly as bright as the coolant dye, but it is visible. A quick Google search makes me think it is the detergent in the oil that is glowing since some detergents fluoresce in UV. I also saw a writeup from a dry cleaner talking about using UV light to identify an oil stain on fabric.