An inpure sample will have a wide melt range and a pure sample will not have??
Correct.
What's a good clear oil??
Crisco Oil, any safflower oil - just go to the grocery store and buy a liter of some likely looking cooking oil.
Capillary tubes are cheap and well worth getting. Using a capillary attached to a thermometer is fine if you don't have a professional device. I wouldn't use the oils recommended in this post for temps over 200 C. You can get silicon oils for that. Technique and a basic understanding of the theory are essential for using melting points as an analytical tool. Here is a link with some info, there are probably better ones out there but this one isn't bad. It has other info too. http://orgchem.colorado.edu/hndbksupport/meltingpt/mtfill.html (http://orgchem.colorado.edu/hndbksupport/meltingpt/mtfill.html)
..oh yeah..you might want to rig-up a magnifying glass to observe the sample so you don't go blind watching it melt ;)