Driven BR30 or Amsoil Break-In SAE 30.
New rings, pistons, valvetrain, etc... requires a break-in, contrary to what some here might think. You can go without, but you'll have better leakdown numbers, more power, and less blow-by using an actual break-in oil and following a break-in procedure. It's also very low cost compared to the price of the engine, and something the engine should be worthy of after going through the effort of rebuilding it.
BR30 contains ~2500 ppm ZDDP that's all branched chained secondary alkyl ZDDP with minimal detergents (<400 ppm Ca), no friction modifiers, minimal VII, and group II conventional base oil.
The type of ZDDP used is more reactive than the typical blended ZDDP in common oils, particularly at lower temperatures. That's important for the first few minutes after initial startup when the oil is still cold. You want a thick anti-wear film established as fast as possible to minimize wear through the break-in. That's what break-in oils are designed to do. The low detergent content also promotes better ZDDP reactivity.
The conventional base oil is used because of its additive response. The additives are more reactive, at lower temperatures, in the lower base oil groups. This helps promote fast ZDDP activation to minimize wear. Group III synthetic base oil is fine as it has as good (sometimes better) additive response as group II conventional. You just don't want group IV PAO synthetics as they are less polar thus weaker additive response.
For the break-in procedure, heat cycling is key. Holding at 2000 rpm for 15-20 minutes on first startup helps ensure good oil flow and splash throughout the engine and gets more heat in the engine faster. Afterwards, shut it down to cool all the way back down to ambient temp. After that, I start it back up, and start making WOT pulls with tuning adjustments, ensuring it gets up to 200+°F oil and coolant temp, and periodically shutting it down to heat cycle.
Like I said, you can go with any oil you want and not do a break-in period, but what would you rather have? A new engine with 5-8% leakdown or one with 2%? One with 3-5 cfm blow-by or one with <2 cfm?
