With the neiros for Chanukah, is there any hiddur with stam oil over candles, or is the hiddur davka with olive oil?
The hiddur of olive oil is primarily because it gives a nice flame, as is written in the gemora. A secondary reason is because it is the oil that was used when the ness of Chanukah occurred. Regarding using other oils there are opinions that there is a preference to use other oils over candles. However a number of poskim seem to say that other oils are not a hiddur over candles, and which ever one gives off a nicer flame then they are better. As a side point, the niceness of the flames is not only in the flame itself, but in the uniformity of the flames. Often when wicks and oil are used, the size of the flames are not uniform, however when using candles, all the flames will be uniform, and therefore nicer.
Shabbos 23a, Tosefos ibid D:H M’raish, Meiri, Kol Bo, O:CH and Rema 673-1, M:B 673-4, Maharshal 85, Pri Chadash -ibid 1, Sdei Chemed Chanukah- 5, Kovetz Halachos (Chanukah) 1-1, Yimei Hallel V’hodaah chap. 14-1 ftnt. 17,18,19 ,20. Chayei Adam 154:8. see Kitzur Shulchan Aruch 139:4