Dye Lite kit
 

I mentioned this in another post and thought I would expand a little. I have no financial interest in either the company or product.

Complete Optimax Jr with EZ-Ject Dye Kit - TRATP-8647


I have used this to find the source of leaks on aircraft engines and on my 911's at various times.

Start with a clean engine. Warm the engine and then wash down in Varsol or other method of choice. Let dry.

Then add dye lite to the engine oil. Go driving and get engine hot so oil circulates. Helps to pick a night without much light. Then hit it with the black light wearing special glasses. Blowing oil shows pale green. Source of leak will be bright green, most likely in a seam between parts.