I finally got fed up with the unexplained oil leak from my kids' much-abused car and decided to diagnose whence it came. I went to NAPA and bought a small bottle of fluorescent dye for engine oil (they make one for coolant, oil, ATF, freon depending on your leak.) This cost $3. Then I went to Walmart and picked up a small AC-powered GE Blacklight for kids' rooms, the kind that emits UV light. This was $10, although I could have bought the big BrightStik for $22 which would have illuminated better. We cleaned the engine at the carwash, poured the dye into the oil, ran it for about 15 minutes and played the lamp around the bottom of the engine while it was on ramps. Sure enough, main seal gone: you could see a dim outline of orange fluorescent dye where the oil was dripping along the block. It outlines the oil trace much better and you can see where it is coming from. Neat.