Where did this claim come from?
The idea that “seed oils” are intrinsically pro-inflammatory and contribute to conditions such as pain and heart disease has been circulating for a while. The mechanism proposed revolves around either that the processing needed to extract the oils for the seed, or that because seeds, like most plant-based foods, contain the essential fatty acid omega 6, or both.
The kernel of truth: processing does matter
In this case, there is real cause to be concerned about the processing needed to extract oils from seeds. This involves heat and chemicals, which can damage (oxidize) the oils. Oxidized oils can potentially be pro-inflammatory4.
A safer option is “cold-pressed” seed and other vegetable oils, which have not been subject to such processing. Probably the “safest” option is extra virgin olive oil, which just need to be pressed. Olive oil has been collected this way for many centuries. However, olive oil is “heavier” than other vegetable oils which may not be preferred for all cooking uses.
In my opinion, occasional use of seed oils is unlikely to be a health risk, especially if what is being cooked contains fresh colorful vegetables, which can contain anti-oxidants to mitigate any potential risks.
On Omega 6 fats
The real misconception here is that the omega 6 fats in the oil are pro-inflammatory. That’s not true. Both omega 6 and 3 are anti-oxidant, which can mitigate effects of inflammation2.
Omega 6 is required for the synthesis of the endocannabinoids and prostaglandins. Whereas some prostaglandins do contribute to the inflammatory response, others are anti-inflammatory and promote healing.
In any case, omega 6 fatty acids are incorporated into cell membranes after you eat them, where they stay until given a signal to leave the membrane to be turned into e.g. endocannabinoids or prostaglandins. So, omega 6 only contributes to inflammation when it is called to do so by conditions in the body, such as infection5.
Follow the money
Sources highlighting the risk of lipid oxidation with plant-based oils, such as seed oils, typically tout saturated fats (such as ghee/butter or lard) as “healthy” alternatives. Often the sources turn out to be meat industry or meat industry-friendly entities, and the objective seems to be to sell more meat-based products. But although saturated fats are more resistant to oxidation than the plant based unsaturated fats, they do get oxidized during heating/cooking as well – it just takes a little longer.
For home cooking oxidized fats may not be as much of a concern as they are for commercial cooking, including fast food. Many fast food restaurants do not change the oil frequently (if at all) so a great deal of the oil they are cooking with will be oxidized after being heated for many hours or even days.