Can an opened can of oil be kept for a certain amount of time before it doesnt work as well protecting your engine ?

I have 3-4 cans of Ford 5w-30 for the missus Zetec-S. When I have done oil changes it only uses 4 out of the 5 ltrs in the car, so im always left with about 1 ltr for topping up but it doesnt use ANY oil between changes .

What i was thinking can they all be pored into 1 can and then used on the next oil change, or does oil degrade after say 12 months if the cans been opened ? I dont want to risk it, if its going to damage her engine.

Cheers for any info esp Simon (oil man) if he reads this