Will a film of oil on water retard evaporation (of water) and prevent air and atmospheric impurities from diffusing into it?
I don't know about oil (fun fact, putting oil on water was a traditional approach to calming waves, thus the phrase "pour oil on troubled waters").  But cetyl alcohol has been used to decrease evaporation from reservoirs, which might be what you heard about.  For instance, here is a report on this, with lots of references.  They used a mix of steryl and cetyl alcohols (long-chain alcohols that will form a monolayer on the water).