You must oil them, or they lose power. I use a Chicago Pneumatic Composite 1/2" impact and love it! Very light, quiet and strong. For bigger nuts, I use an Air Cat 3/4". Weighs a ton, but is also the quietest air tool I own. When the Air Cat won't do it, I break out the Torque Multiplier "Nut Cracker". Uses a hand crank and gearing that multiplies your power X 58!!
Look it up on YouTube. Type in "torque multiplier". Works exactly as shown.
I have had good luck with the Chicago Pneumatic it is 20 years old and has had the s--- worked out of it, You do need to oil it and keep water out of it.