A liquid that forms a surface membrane that prevents evaporation but allows oxygen to pass through
I don't think you or your cells want any strange low molecular weight substance in your experiment.

You can buy polymethylpentene (PMP) foil, this stuff has a rather low density compared to PE and other polyolefines, and therefore a high permeability for gases (10x higher than PE for oxygen, and PE is already not exactly tight against gases, x20 higher permeability than PVC for example). 50µm films are readily available, you find them online in a minute.