I want to know if it is true that using olive oil is a Sunnah of the Prophet (sws).
No this is no Sunnah of the Prophet (sws). As a principle, the Sunnah is that established practice of the Prophet (sws) which he sanctioned as religion. Acts which are not done in a religious perspective never form the Sunnah. In other words, using olive oil or such other acts of the Prophet (sws) like riding a camel or eating by hand or wearing a turban are related to the common practice of the society he lived in; for example, he fought with a sword because wars in that time were fought with swords; he used a Miswāk stick because this was the common way of cleaning one’s teeth in those times; no one can say that if today a person uses a gun to fight a war or a toothpaste to clean one’s teeth, he would not be following the Prophet’s Sunnah. This of course does not mean that a person cannot adopt these practices; he, however, should always be clear that these are not Sunnah.