Toasting spices with and without oil
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.