if you have more than 4000 miles on the engine since rebuilding it Synthetic is the best
the problem with new oils is the lack of Zinc & phosphorus , Royal Purple & Redline have high quantitys of both , I have been using Castrol SYntec 5-50 for a long time as well. You can also add the Zinc & phosphorus by adding Gm EOS or Comp Cams has an additive as well