I'm trying to track down and confirm a potential rear main seal leak. I bought a 1 oz bottle of Tracer Products LeakFinder leak detection dye for oil-based systems (Link) and added it to the engine oil. After cleaning the area really good with engine degreaser, letting it dry, and taking the car for a drive, I can see the area starting to darken as if some oil is seeping. However, when shining a UV LED flashlight that I bought from Amazon (Link) on the area, I don't see it glow fluorescent yellow/green as expected. Shining the light in the empty bottle doesn't glow either. I don't need some kind of special light, do I?