Can I reuse cooking oil after it has cooled down?

Though people do, it isn't advisable. Among other potential problems, used cooking oil has been linked to the formation of carcinogenic compounds at high cooking temperatures. Additionally, a cooking oil's smoke point--meaning the temperature at which it begins to decompose-- diminishes each time a cooking oil is re-used. As a result,  used oil may become unpredictable during cooking. Finally, used cooking oil may pick up unwanted substances and odors that can damage the flavor of the product you are cooking. 

 
Source(s):
Thirdage.com "How Safe is Cooking Oil?"
Cheftalk.com "Safe to Use Old Cooking Oil?"