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Castrol is good oil. I understand that you are concerned about the low levels of zinc and phosphorous in the current-formulation oils during break-in---especially with our flat-tappet motors. My choice would be to run the Castrol conventional oil with GM EOS (or AC Delco EOS - same stuff), or run Shell Rotella diesel oil which also has higher levels of Zn and P.
On my 4.0 I used the straight 25wt Pennzoil racing oil that my local engine builder supplied. He told me to start it up, run for 20 min and shut it down, let it cool and then drive it like I stole it for the first 1k. Then change the oil to whatever I wanted to run except synthetic. He wanted me to run it a year on dyno before I put in the syn.
I followed his advice since he gave me a 2 year warranty on the engine. The warranty is almost up and the engine is just getting "broke in" good. It runs great.
We have used Brad Penn's Break in and the Joe Gibbs break in with good results. Whatever oil you choose to run make sure it has ZDDP in it, it is vital to flat tappet cams and lifters.
Castrol is good a base break in oil... But it has been reformulated to meet the lower Zn and P numbers for the SM spec. So you absolutely need to add some type of break-in additive to it... GM EOS would be a good choice.
Follow the instructions your engine builder/rebuilder gives you. If they tell you to use only a certain brand, that is a violation of Magnasson-Moss act.