I would not reuse that oil. The frying-oil reuse that occurs in fast food chains is reuse for the same day or over just a few days. They also store it properly and not just sitting exposed in the fryer.
justkt is correct that more saturated fats are less sensitive to breakdown, this is why bacon grease can be kept for a rather long time.
Three factors play a significant role in oil degradation:
Oxidation
Any contact with air causes oxidation in oil. High temperatures, metal alloys, surface exposure, and even UV light act as catalysts to this reaction.
Hydrolysis
When water interacts with oil it causes it to taste tainted or acidic. This is exacerbated by high temperatures, heating/cooling cycles, and oxidation products.
Polymerizaton
When frying oil deteriorates, the resulting products form both volatile (or reactive) and non-volatile compounds. Non-volatile compounds remain within the frying oil, and can produce polymerization at frying oil temperatures above 200°C (390 F) or in isolated hot spots within the frying system. These molecules bond together to form large, different-sized clusters that accumulate on the oil’s surface. Since they don’t dissolve, they cause foaming; trapping air under the oil, and increase the possibility of hydrolysis.
Given all these, the best environment for any oil is an cold, dark, airtight place. Cold temperatures may cause clouding, but this is not a cause for worry.
Source:
http://www.heatandcontrol.com/technical%20articles/Maximizing%20Cooking%20Oil%20Life.pdf
I would recommend borrowing from the principles found in this post, which details how to "fry" chicken in a kettle grill: Kettle Fried Chicken.
Two key takeaways here: 1) high heat, and 2) all (or nearly all) your food's surface area is exposed to heated air. You'll just replace "kettle" with "oven," and put something under the rack on which you're cooking your food to catch the stuff that drips away from it, and you should be good to go. It may require some experimentation to find the right mix of time and temperature for the food item you wish to cook, but it shouldn't take more than a few attempts to get it right, if not the first one.
Best Answer
The trick is to chill the Mars bar in a fridge for a few hours before cooking.
Prepare a batter mix (the kind you deep fry fish in) and get your oil heated to temperature. Here's a batter recipe I've used before:
(You can't fault Delia!)
You can use sparkling water to introduce more bubbles into the batter which gives it a nice texture.
Remove the Mars Bar from the fridge and immediately coat in the batter and cook in the oil right away. Cook until the batter is golden and serve with a scoop of ice-cream.
You can also do this with Snickers bars. It's also worth trying with miniature Mars and Snickers bars.
As a Scotsman I must apologise for inflicting this culinary delight on the rest of the world :)