After heading to my trusty Indian market yesterday I was speaking with the owner about how awful fenugreek seeds are uncooked. He suggested (with black mustard seeds and cumin seeds) to throw them in the pan with a little bit of oil. I've read much about people extolling how good the dry heat is for the spices. Personal experience tells me that adding oil to the pan works better. Does cooking with oil change the flavor of a spice while toasting? Are there any spices that it is important to toast without oil?
I think it is mostly preferential as you can toast either way. Using oil can be tricky though as there is a fine line between toasting with oil and frying your spices. An article I found boasts using no oil which I like. There is also a list of spices that benefit from Spice Toasting. I do believe toasting with oil can change or influence the flavor depending on the oil used.
Indian recipes seem to always call for oil.
Non Indian recipes seem to always call for dry (or don't toast at all- yuk).
In my personal experience (mostly mustard, cumin, fenugreek, coriander)- dry toasting calls for more of my attention. I have to have an appropriate temperature, agitate the seeds, and watch for the telltale aroma to know when they are done. If I don't pay enough attention to any of those then they will scorch and be horrible.
With oil I have more leeway. The seeds don't need to be agitated as much after they are coated with oil and they pop audibly when they are done. It may be my imagination but the oil itself seems to distribute the flavor of the seeds better as well.
I have scorched dry toasted seeds on occasion when I was distracted. I have never had oil toasted spices not turn out perfectly.