1970 Pontiac GTO break in oil and fluids

First, being that you say the cylinders were within.001 of factory tolerance -- What machine work was done to the cylinder walls -- re-honeing or re-boreing? Did you change the piston rings? If so, what material are the rings? In order to seat the rings properly, I must know exactly what was done to the cylinder walls.

The important thing as it relates to engine break in, and oil, is the cylinder walls and the piston rings. These are the parts that really MUST be properly addressed -- everything else is important (don't get me wrong). First realize this, the cylinder walls and the piston rings are NOT perfectly mated (yet). They have microscopic ridges or bumps on them that must be "worn" off. This is what is called break in. The break in, perfectly matches the cylinder walls to the piston rings so you can have the best compression possible - and this period will be VERY important (to you especially) because is seems that you will not be putting many miles on this engine.
An important step, BEFORE you fire the engine for the first time, is to pre-lube the engine. This is done by inserting a tool, connected to a drill, into the distributer hole. I have a Comp Cam pre lube tool (part number 4921) they are about 25 or 30 bucks. This will ensure that you have oil distributed properly BEFORE the initial start. OK - now to answer the question --LOL -- I suggest using straight 30 weight NON DETERGENT (if you can find it) and NON SYNTHETIC oil for the first to 800 to 1000 miles. The breaking process MUST be done as follows: Do not go over 60 MPH for the first 1000 miles. In these 1000 miles, vary your speed and drive up and down hills (if possible) often. Do not drive at steady consistant speeds. Do not "lug" the engine by driving in high gear. The reason for this is to vary the loads "gentaly" on the piston rings. If you load them too hard or too soft you will not get a good seal and it will result in smoking and/or excess oil consumption.
Also, drive these 1000 miles as soon as possible. Not all at once, but do not just drive on Sunday and every once in a while. It is important not to let oil drain off of the cylinder walls and condensation to accumulate in the crank case, which is what happens when you let any engine sit for a long period.
After the 800 to 1000 mile break in period, you should use 10w30. Also, if you want, you can use synthetic oil at this time. I do suggest switching to synthetic for reasons I will discuss, if you want. I further suggest using either Mobile 1 (not just Mobile) or Castrol oil brands -- for reasons I will also discuss if you want.

BTW - Im sure you replace the oil filter every time you replace the oil, but if it is possible and not too messy, try to replace the filter only after the first 200 miles. This is not really necessary, but I do it and I have found (when inspecting oil filters) excessive debris in it from the rebuilding process. If you do - do this -- be quick about it, it is a little messy, and be sure to add a little oil afterwards.

One more thing, change the oil more frequently, being that you are putting low miles on this engine. Every internally combustion engine accumulates condensation (water) inside of the engine. It is a result of the heating and cooling of the engine and there is no way to avoid it. This gets into the oil. This water usually does not cause a problem because it is evaporated effectively during the next engine heat cycle, which in most cases is, the next day. It seems that you will not be consistantly running the engine every day (after the break in period) so change the oil more frequently -- maybe every 2000 miles or at least twice a year (before storage and before long storage periods in the winter is best)
Factory suggested oil service periods of 3000 or 5000 are assuming that the car is being driven frequently.

Sorry about that--
There is no additive that should be put in the oil at break in. However, when installing the cam, it must be coated with cam shaft assembly lube during installation.
As far as the zinc content of oil goes, your research has a little merrit. The most important thing about oil is its' ability to carry dirt and debris away from the bearing oil "wedge". This wedge is measured in thousandths of an inch - (bearing clearences). If this wedge is interrupted for any amount of time - while the combustion is taking place - and the pistons are pounding down on the crankshaft -- the engine will eventually fail.I suggest switching to fully synthetic Castrol Syntec 10-30 or Moble 1 10-30. These two oils -IMO- have the proper chemical compounds/detergents/and debris suspension properties that are the best for any engine.