Ok here goes. I have just purchased an 80 chevy 3/4 ton 4x4 pickup with about 130,000 miles. All my vehicles are newer with roller lifters and use Mobil 1 synthetic. However, this 350 is older with flat tappetlifters. What oil would be good to use? Here in IA temps range from -1d to +100. 10w30 is good in winter with 10w40 or 20w50 in summer. Will running synthetic oil possobly cause leaks to start? I have heard rottela 5w40 synthetic. Won't cause these leaks, but I'm not posotive. Running a dino oil is no problem, but I desire an oil or even an additive that would be benificial to flat tapprts.