Is there any source to the minhag of lighting oil candles on Shabos?
Yes there are numerous sources for this, here are some. See Shulchan Aruch O:CH 264-6, Magen Avrohom ibid 12, Oruch Hashulchan ibid -8 that olive oil is preferable to using wax Also see Mishneh Halachos 5-41. Horav E. Greenblatt zt”l told me “that a person that used to light with olive oil, and then switches to candles should be matir neder”. Therefore if someone is going to start lighting with olive oil she should say that it is bli neder. The main reason for this is because olive oil makes a nice flame. Others challenge this and say that our candles nowadays make a very nice flame, and therefore are better than olive oil, because there is no fear that one might tilt the cup of oil to make the fire bigger. See M:B ibid 23.