Mnemon: Yes, but likewise most grapeseed oil 'ordinarily bought' is heavily refined; I already pointed out the quality of oil is important; that, too goes for ANY type of oil. You can get light olive oil that's fine.
You can get bad grapeseed oil, depending.
Perhaps it's possible to come across grapeseed oil that isn't of the best quality, but saying that
most grapeseed oil is heavily refined is definitely incorrect. I've been buying grapeseed oil for years and every bottle that I've ever bought clearly stated that it was "pure" grapeseed. It's not as common as olive oil so theres not as many brands to choose from which means the quality doesn't vary nearly as much as olive oil does.
Mnemon: The important aspect is, between just these two oils that the very base composition of grapeseed oil as shown above, doesn't react to heat well. Olive oil is better - it's not the best monounsaturated oil out there for frying, but grapeseed just doesn't take heat well in comparison. Both release cancerous aspects when they reach the smoking point so that's something to avoid. Yes you can reach higher temperatures with most readily available grapeseed oil before it smokes; the way it breaks apart makes it less healthy in comparison.
If cooking with low-mid heat then yes I'd say cook with olive oil over grapeseed all day long. But with respect to high heat cooking, which is the type of cooking I've been talking about all along, I totally disagree. I'd much rather use grapeseed oil even if it's predominantly polyunstaurated than use olive oil that's going to smoke. Once the oil starts smoking, it turns rancid and should not be consumed
at all. Even breathing in the fumes of the smoke is harmful which is worse than the polyunsaturated content of grapeseed. Also, lets not act like olive contains
no polyunsaturated fats. Lets also not act like grapeseed oil doesn't contain a fair amount of monounsaturated fat.
Mnemon: And as I also mentioned earlier - despite whatever oil you use it is
sensible to use lower heat and longer cooking time IF health is important aspect of how you cook. High heat for frying / cooking = bad. Most cooking / frying doesn't need to go beyond 350 degrees Fahrenheit. Most works fine even below that. It's important not just in terms of the reaction of the oil to heat, but what higher temperatures do to whatever you are cooking.
Of course it's ideal to say people shouldn't cook over 350 degrees because that's around where the smoke point of olive oil is, but it's also very unrealistic. Most stovetops don't even allow you to measure the temperature at which you're cooking. They usually just measure heat intensity with numbers such as 1-10 or something along those lines; and using 6-10, which pretty much everyone does, is more than likely going to smoke olive oil every time.