Minimum amount of oil that should be used in deep fryer
If you need a larger basket to fry the amount of food you want to cook, you need more oil as well. If you don't always want to make that much food (or use that much oil) then you need a smaller vessel for deep-frying, such as a second fryer or a small wok or kadai. 

If you put less oil in than the minimum it may not cover the food and/or the oil could overheat, depending on how the fryer manages its temperature settings.

Overheating oil can be dangerous and will also burn your food while leaving it raw inside. All around, something to avoid. 

You can save and reuse frying oil. There are some good tips at the sites below.


Let it cool fully then filter out food particles (I use coffee filters and a funnel for this; it can be a slow drain, be patient)
Refrigerate used oil to retard bacterial growth 
Stop using it if it looks weird or smells funny!


http://www.seriouseats.com/2013/09/ask-the-food-lab-how-many-times-can-i-reuse-fry-oil.html

http://extension.psu.edu/food/preservation/faq/questions-and-answers-about-re-using-frying-oil

http://www.epicurious.com/expert-advice/you-can-reuse-frying-oil-article