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: