Some advocate (me included) using a lighter oil for break in, the reasoning behind that being flow. That would be 10W30 in our case. Flow's important for several reasons. You want good flow at first start and good flow thru the first warmup so as to flush out contaminants (there is a lot of dirt in the engine after a rebuild) as well as flushing out break in contaminants for those critical first few hundred miles. Also, because you probably aren't going to put those first few hundred breaking miles all at once (500 is a good number BTW, unless you have a bypass oil filtration added, in which case you can run a normal interval) you don't want to have the filter bypass... you want ALL the oil running thru the filter. On cold/cool starts or hard running, cool, thick oil can cause the bypass valve to open and allow unfiltered oil to bypass the filter. Break in oil is not the time you want that to happen because that's when the oil is likely to be full of junk. The important times are starts, specifically cold starts. The optimal thing is to put as many of those break in miles in as big a chunks as possible on a warmed up engine... in other words, it's better to break it in in 100 mile increments than 10 mile increments because that eliminates 9, cold, dry starts. It's not always possible or practical to do that but bear it in mind. If you eliminate many cold start aspects and can do the break in in big chunks, it reduces or eliminates the need for the lighter oil... assuming you want to run 15W40.... because the oil is warm and flowing good. Some of us use 10W30 all the time. ( : < )
Since I "retired" from being a full-time tech, where you were on the clock, I now take some extra steps at first start most of my bosses wouldn't allow.
1) Don't fill the cooling system until right before first start and then use hot tap water. I never fill the engine with coolant the first time because there is junk in the water jacket and elsewhere so running water for the first run and maybe the first run around the block (or in my case the first hundred miles) will flush that stuff loose and you can dump it. Also, if you end up with a little leak, it's a lot easier to deal with a little water leaking than expensive coolant. There is often some oil in the water jacket so a mild flushing compound might be in order too.
2) I also warm the oil by putting the jugs in a bucket of hot tap water, warming up the water frequently to get the oil at least as hot as the tap water (takes 30 minutes and about three refills of hot water). That helps the oil flow better for the first start.
3) If the oil and coolant or water are already in, plug in the block heater for a few hours and maybe even let a heater blow on the oil pan (a safe heater!). It was cool out when I was breaking my truck in so I plugged it in every time for the first few hundred miles. There is some heat transfer from the water jacket to the oil (I tested it at 45F ambient and with the truck plugged in for two hours, oil temp in the pan rose 30 degrees) and on top of that the oil cooler also warms the oil so the hotter the coolant at start up the faster the oil will warm up.
As to a specific oil, I like a break in oil with high ZDDP and even a little moly. Diesel oil is higher in ZDDP than gassers, but they still reduced the levels for the CJ-4 oils and changed the types of ZDDP used. I am not normally a fan of oil additives, but break-in is a good time for a can of a ZDDP additive. ONE can. Also, if you choose an oil with lots of moly (molybdenumdisulfide) it's very complementary to ZDDP. ZDDP needs some heat to start working where moly works well when cold and, if anything, starts to back off with more heat. The Delo, Delvac and Lubrigard are three that come to mind, the Delo has 92 ppm/ml and the others about equal levels at ~50 ppm/ml. There are many ZDDP additives and I don't have a specific recommendation. I can tell you what I used, Rislone Engine Oil Supplement with Zinc Treatment. Here is the data sheet <http://rislone.com/wp-content/uploads/2014/11/4405-Tech-Sheet.pdf>. What I liked is that a) it was everywhere, b) it was less expensive c) it had a little less ZDDP than some of the more expensive stuff, but just enough to bolster the oil, d) it didn't change the viscosity of the oil (like STP or motor honey).
PS- Synthetics are OK for break in, most OEs put syns in for first fill these days, but my objection is cost. Who wants do dump $10/quart oil in 500 miles? An exception would be if you have a bypass system with a fine filter. I have nearly 8K miles on my break in oil. I have tested it twice and with a 99.9% @ 20 um primary filter (Fram Ultra) and a 3um bypass filter, the oil at 6K miles had fewer contaminate in it than the new oil (yes I tested the new oil). Most synthetics these days are Group III, which are less "slippery" than the Group IV and V oils we used to call synthetic. I use the term "slippery" to save a long explanation. Many conventional oils are Group IIA base oils, which are really only a few steps below Group III and, frankly, if you were worried about a Group III for break in, a Group IIA is almost as much to "worry" about. Plenty of people break in engines on stuff like Amsoil and the other boutique oils. The main issue used to be for ring seating but today's rings and bore finishes (if properly done) don't have a problem breaking in on a syn. Again, it comes down to dumping #10/qt oil in 500 miles!
Also, now is the time to install the most efficient oil filter that you can afford. Not only will the oil be full of junk from the rebuild, it will generate more junk as it breaks in. The more you catch, the better and the high end filters are more efficient, such as (in no particular order) the Mobil 1, Fram Ultra, Amsoil, Napa Platinum, Purolater Synthetic, and... uhhh I'm drawing blank on the others. Not all the filter manufacturers offer a high efficiency filter in either FL784 or FL1995 equivalents (easier to find a hi-efficiency filter in an FL1995 equiv.). No matter what, at least buy the FL1995 equivalent filter, whatever brand. It's a little more efficient than the FL784 but most importantly in this case, it has about twice the filter area. That's important to good oil flow, especially at first start and cold starts.... less likely to open the bypass valve.
