Subha, yea kaisa post hai? lol!!!
Don't you know Sarson ka tel? It's Mustard oil and it's not for human consumption, at least not in such bulks as Mohan seems to have drunk (I haven't seen the episode yet but Indu called him "Devdas" so Mohan must have consumed quite a lot). Mustard oil is only used in little amounts in cooking or for body massages.
So Guru said "It's Sarson ka tel" to prevent Mohan or rather stop him from Drinking it but Mohan obviously knew better..
One more info about mustard oil, it's got a strong characteristic too (due to which I think, Guru really said Sarson ka Tel and not any other thing). That characteristic is - When mixed or added to alcohol, it makes the alcohol in-edible, poisonous..you can call it de-naturing rather.
So Guru came up with this oil specifically to stop Mohan, but Mohan must have caught him and his lie, that's why he went onto drink.