low oil pressure

I had low oil pressure 20 pounds and sometimes lower. I had the Oil pickup tube replaced and the pan and screen cleaned. While cleaning the pan my mechanic discovered the oil pan was removed before and the pan was cracked. He said that when the pan is put back on that you have to follow the torque sequence or you run the risk of cracking the pan because it is cast aluminum. The o ring replacement got my oil pressure back to 40 psi. My truck has 228,000 miles on it. The o ring was flat on the outside, so if you have high miles and low oil pressure and the pressure switch is good change that oil tube o ring. My truck is a 2002 Silverado 1500 extended cab 5.3L LS.