My engine had a tiny, mysterious oil leak. After each flight the rear of the engine had a thin film of oil on it. Not always enough to drip, but enough to feel or to see on a clean rag. I have never been able to find the source of this annoyance until this weekend.
I found these two products on Amazon.com and they worked like magic. An ounce of the dye in the oil tank, a brief flight, then using the ultra-violet light in a dark hangar the leaking banjo fitting lit up and was plain to see.
The dye cost US$6.22, the flashlight US$7.25 You can order them from these links:
400 Nm Uv Ultra Violet Blacklight Flashlight
Interdynamics 374CS Engine/Transmission Dye
I found that this system works best with very clean oil in the engine. I drained my oil, refilled the engine with the cheapest oil I could find in the auto parts store, ran it up to temperature, drained it and then refilled the engine a final time with the correct grade of aviation oil before adding the dye. The oil glowed brightly under UV light. If the oil is dirty, the carbon particles in suspension will absorb/block much of the light and the process is less dramatic.