What effect does heat have on cooking oil shelf life?
As I know almost all cooking oil become unhealthy and toxic when the "smoke point" is reached. Every oil has a temperature point that should be never reached, as follows:

For mechanic extra virgin oils:


flax, safflower and sunflower oil - 107 C
walnut and peanuts - 160 C
coconut, soy, corn and sesame oil - 177 C
extra virgin olive oil - 190 C


The regular cooking oils have "smoke point" even higher like over 235 C so it's better to use those for cooking and let extra virgin ones for salads and cold dishes.

Or just be aware to not see smoke while cooking with oil:)

Me, I'd suggest butter instead, because at higher temperature butter turns to "ghee" which is quite healthy.
 - avocado oil - 190 - 204 C
 - almonds and grapes oil - 216 C
 - palm oil - 235 C