How to keep my (deep frying) oil usable as long as possible?
First thing, a cold zone doesn't prevent particulate build up, it just cuts down on the amount that will remain suspended in the oil. You should still filter if you want to keep your oil as long as possible.  You can detect bad oil (meaning rancid) by a few ways, like smoke point or smell.
You should store your oil someplace dark and cool, which could be the inside of your fryer as long as you filter and clean the sides before you stick the oil back in.  Another thing you can do to extend the life of your oil is switch to tallow (beef fat) or rended pigs fat (lard) as saturated fats do last a lot longer than the poly-unsaturated. 

Once again, you should lose the oil when the smoke point drops, or if it is to dark, or if it develops any foul odor.  And as I said before, once you use the oil once, you should NOT use it past six months under any circumstances, if you do, you are taking your gastro-intestinal tract into your own hands.