Evaporation, oxygen and any number of other effects won't change the ratio of U235 and U238. Those isotopes are chemically identical. The only way they are different is by their different atomic mass, i.e. their gravitational effects. And since the atomic mass is different by only about 1%, even the gravitational effects are almost identical. So U235 constitutes between 0.71% and 0.72% of the Uranium everywhere in the world.