You can explain away the oil monolayer experiment with surface tension. Anyway, this experiment works best if the oil is dissolved in a large quantity of petrol

You can explain away the oil monolayer experiment with surface tension. Anyway, this experiment works best if the oil is dissolved in a large quantity of petrol and it's fairly certain that the Greeks did not know distillation. More compelling would be Brownian motion. Glass was known in the Hellenistic era, and the Romans were expert stonecutters. The principles of optics were known to the Ancient Greeks, and some guy could have made lenses and assembled them into a microscope. Unfortunately that path was not taken until the 1600s, but the technology was all there.