פירוש על עבודה זרה 70:21

According to Rav, Daniel (from the Bible) made a decree against using Gentile oil. While the Talmud does not explain here what Daniel’s reasoning was, we can assume that it was to prevent Jews from fraternizing with non-Jews. In other words, the prohibition of Gentile oil was a step to keep Jews from assimilating with non-Jews.
Shmuel seems to fear that the oil actually has prohibited ingredients in it. While there is nothing wrong with the oil, if the oil was used in vessels that had been previously used with forbidden foods, then the oil is forbidden.