It's good to check current status occasionally, as the official guidelines change so frequently. Especially with regards to dietary fats there have been lots of changes in public dietary guidelines recently and in my opinion, they still haven't caught up with the research and there seems to be a lot of confusion. The only thing experts seem to agree on when it comes to fats is that trans fats (partially hydrogenated vegetable fats) are bad. Well, mostly. Apart from CLA, a naturally occurring trans fat, very different to the ones produced by hydrogenation.
There's lots of articles around on healthy fats, but often these just assume that you're using your oil raw. If you're going to heat your oil, the criteria for choosing the best one are quite different. Basically, heating fats is never healthy. It will always make them more unhealthy, but to varying degrees. The more and the longer you heat your oil, the more unhealthy it will become. So you're not looking for the healthiest fat, but for the least harmful, as a sort of damage limitation. And obviously it's best to keep heating oils to a minimum. The safest oil for prolonged high temperature cooking is probably avocado oil, but it may be difficult to find in the shops. You're looking for a low smoke point and low PUFA (polyunsaturated fatty acid) content. PUFAs are essential fatty acids like omega 3 and 6. They are unstable and produce free radicals when heated. So saturated fats and MUFAs (monounsaturated fatty acids) are generally safer for heating. Saturated fats are mainly animal fats, ie from meat or dairy. There are two plant contenders – coconut oil and palm oil. Palm oil is being phased out, not sure if it's already disappeared or is due to disappear soon, due to its environmental impact of causing large scale deforestation in tropical regions. PUFAs are found mainly in vegetable oils, and MUFAs in both animal and plant based fats. Here's a bit more information about the smoke points of oils.
So macadamia or avacado oil would probably be the best choice for chip shops, although not the most realistic one. Wouldn't avocado chips be tasty though?! But for cooking at medium heat or for a short period of time, as is usually the case in home cooking (unless you're re-using your oil), any oil is probably safe to use, but would depend on the amounts. Most things are safe enough in moderation. It also depends on the exact temperature you're heating it to, and the amount of time it stays at that temperature. I think it becomes clear that it's not black and white, but more of a guessing game. Having said that, olive oil has a lot going for it with its low PUFA and high MUFA content and releatively high smoke point, even higher when refined, ie not extra virgin. Although refining brings with it a whole new set of drawbacks. Plus, olive oil has a long history of traditional use in all types of cooking which is always reassuring. Unlike rapeseed, which was only made suitable for human consumption in the 70s. But as rapeseed (called canola in the US) has become quite commonly available recently, it's worth exploring a bit more. It's not a bad oil at all, going by its credentials. But It's inferior to olive oil in cooking. True, it has a marginally higher smoke point, but a much higher PUFA content. And there's also the small matter of taste, which I find is much underrated these days. Personally I'm not keen on the taste, and I don't like the big yellow headache fields either! But that's just my personal view. Here's a little article on rapeseed oil. And here's one on olive oil for cooking.
Sunflower and safflower oil might be good cooking options too, but I find them too confusing. They each come in a range of qualities with very different smoke points and PUFAs, depending on the level of refinement and possibly variety too. Not sure if there's a way to find out which type is in the bottle. Palm oil would actually be a very safe cooking oil, if only it could be produced sustainably.
When we talk about damage limitation – what sort of damage are we talking, exactly? Heating fats beyond their smoke point results in the release of acrolein and free radicals from oxidation. Acrolein is an irritant and responsible for the acrid smell of burned fat. Free radicals cause all sorts of trouble, being implicated in many of the main degenerative diseases like stroke, heart disease, diabetes and cancer. It does not, however, produce trans fats, which can only be produced under industrial conditions. Oxidation and free radical release is the same thing that happens when fats go rancid. It can happen at relatively low cooking temperatures if the oil is high in PUFAs and hasn't been refined. So free radical damage and the acceleration of degenerative disease over time is what we should worry about. There is good news though! Free radicals can be held in check by antioxidants. You'll find antioxidants in pretty much all fruit and veg, and of course herbs and spices too. Again, it's really not just black and white because some free radicals are necessary, and antioxidants are plentiful in our diet. It all depends on quantities and ratios. But generally, as long as you eat fruit or vegetables with your meal, you'll make up for a certain amount of potentially harmful free radicals.
Here's a really long but comprehensive article describing the differences between fats, in case you want to know more. It mentions all the things left out in this article, like refining, rancidity and flaxseed. I recommend skipping the first chapter as it's just about another article which isn't very interesting unless you've read it.
Here's a table with the composition of culinary fats.
So to sum up – do use the so-called healthy oils (the ones high in PUFAs), and use them often, but for god's sake don't heat them! Or at least, only heat them gently. If you're going to fry the hell out of your fat, then choose the formerly classed as highly unhealthy fats – ie the ones low in PUFAs, high in saturated fats or MUFAs and with a high smoke point, like butter, ghee, lard, coconut oil and also olive oil. ...And eat your greens!