Used motor oil works great. Just place on car body in drops about half the size of a penny and drive 70 mph for a mile or two.
These were some oil tests I did on my WRX on I70. I would stop on the entrance ramp shoulder, put the oil spots on the body, then flog it up to 70 mph and pull off at the next exit (usually a mile away outside of town) and take the pictures. Wipe the oil off, then turn around, apply new oil and head the other direction back to the original location.