Viscosity, temperature, and composition all play a part in the cooling rate of an oil. If it is an issue in your heat-treating, perhaps you should consider oils designed specifically for oil quenching such as item numbers 3202K1 and 3202K7 in McMaster-Carr. These are the part numbers for a 5-gallon container of normal speed (28-second quench time) and high speed (10-second quench time) respectively.
http://www.mcmaster.com/
You can see that the difference in cooling rate is significant. However, it is a moot point if you don't know your metal composition. Every metal type has a recommended heat-treating chart with guidelines for optimizing that alloy for a particular application. Guessing is usually futile.
For example, here are two pages on M2 steel:
http://www.suppliersonline.com/propertypages/M2.asp http://www.pvsteel.com/docs/Tsb-118.pdf
Here's an example of the real complexity involved in crystal transformation in heat-treating:
http://sb2.epfl.ch/instituts/Gotthar.../Research.html You can see that it is not trivial. If you are a blademaker or machinist, then you pick your alloy, and learn its particular heat-treating demands, including the best quenching composition and temperature.
On this table,
http://www.knives.com/heatreat.html you can see at the bottom the relative shock rate of different quenching mediums. It is interesting to note, that the agitation (how fast you move it around while quenching) has a very big impact on the cooling rate. You can see that according to this table, the shock rate of oil is about 1/3 that of water... all other things being equal.
Having said all this, if you are a ornamental blacksmith making ordinary shop tools, then use a clean vegetable oil in a can with a drop-down lid and go forge something. The difference in quenching behavior between oils just isn't significant enough for us.
A can like this:
http://www.barcoproducts.com/store/i...ARTMENT_ID=111 with 5 gallons of vegetable oil has done me fine for years. The vegetable oil does not go bad. The lid keeps the bugs and mice out, and the fire in. The foot petal leaves your hands free.