My mother always used to say that a cup of cooking oil would fix anything that ails you. And now, I have a plumber who makes the same claim when it comes to sealing off an open drain where the existence of a P-Trap is questionable.
Both he, and my Mother's common statement is "Pour a cup of cooking oil down your Trap!"
Oil always sits on top of water, thereby any gases (sewage) are stopped from rising above the water. ================================ There was a recent case in the USA where a whole building was evacuated because of "gas leak". It turned out that the Urinal cakes, one to a urinal, was doubled up to improve the cleanliness and also to halve the service time, by the janitor. Too much of one thing doesn't make it better.