Okay... some nuggets from Shirley. Cookwise is an AWESOME book, and her section on frying is fantastic.
"New crops like high oleic sunflower oil and low linolenic soybean oil maximize single double bonds and minmize double and triple double bonds, making for more stable and healthful oils. The more healthful unsaturated fats can be used if the oil is not going to be re-used. Considerations like flavor and smoke point may be more important than saturation." (158)
That's basically why I keep HO sunflower oil and tallow on hand. Tallow is super stable (and not terrible for you, if your cow didn't spend the last two months of its life in a concentration camp). But for a vegetable-based option, HO sunflower oil is pretty dang good.
She does not recommend re-using fry-oil from home because it does not contain the common additives (like anti-oxidants) that help keep commercial fryer oil from breaking down. I don't fry that hot and I use relatively stable fats, so I don't know that this nugget applies across the board. You can always add some mixed tocopherols to your oil if you want to, and create your own "commercial" fry oil.
Commercial fry oils contain trace amounts of certain silicones, which form a film on the surface of the oil, preventing direct contact with oxygen in the air (and thereby limiting oxidative rancidity).
Solid vegetable shortenings often contain emulsifiers like mono and diglycerides, which makes them good fats to use in cakes but lowers their smoke point and makes them bad for higher temp frying.
I couldn't find anything on polarity and browning, but KennethT is right on the money that you don't need to add much "old" oil to fresh oil to reap the benefits of slightly damaged fry fats. The key is that the damage is *slight*. You don't want to slop back a bunch of burned up fishy-smelling rancid garbage oil into your jug. That's not going to be good for anyone.
