Edit History

I looked through some Indian cookbooks and while urad and chana were by far the most commonly used dals for tadka (the term for fried spices regardless of when they're added during the cooking of the dish) , I did find a few instances of toor dal being used that way.  

Also, some of the sambar powder recipes in the cookbooks I examined used toor dal as an ingredient, and the sambar powder recipe in Madhur Jaffrey's World Vegetarian has moong dal as an ingredient, so it appears it's not just urad and chana dal that are used as spices in South Indian cooking.

edit: I bought World Vegetarian a few days ago and am still looking through it, but I ran across a recipe where Jaffrey offers the choice of urad dal, chana dal, or yellow split peas in the tadka.  

I looked through some Indian cookbooks and while urad and chana were by far the most commonly used dals for tadka (the term for fried spices regardless of when they're added during the cooking of the dish) , I did find a few instances of toor dal being used that way.  

Also, some of the sambar powder recipes in the cookbooks I examined used toor dal as an ingredient, and the sambar powder recipe in Madhur Jaffrey's World Vegetarian has moong dal as an ingredient, so it appears it's not just urad and chana dal that are used as spices in South Indian cooking.

I looked through some Indian cookbooks and while urad and chana were by far the most commonly used dals for tadka (the term for fried spices regardless of when they're added to the dish) , I did find a few instances of toor dal being used that way.  

Also, some of the sambar powder recipes in the cookbooks I examined used toor dal as an ingredient, and the sambar powder recipe in Madhur Jaffrey's World Vegetarian has moong dal as an ingredient, so it appears it's not just urad and chana dal that are used as spices in South Indian cooking.