what the heck!!!!

My mom bought a Ford Taurus with the 3.0L V6 in it a few of years ago and as it said in the owners manual to put 5w-30 oil in it. So we did and the thing burned 2 quarts every oil change. So next time we put 10w-30 in and it never burned a drop in the next 5 years. Not to mention it only had 29,000 on it when we bought it. Now I have bought a 94 ranger with
the same motor and it does the exact same thing. Why the heck do these engines burn oil with the oil it is recommended to put in them???