My personal rule of thumb is second regularly scheduled oil change, switch to syn. You will likely be fine now as the ring package is so well tuned that as mentioned, many come off the showroom floor with a blend at minimum.
It used to be that it took about 8-10,000 miles for the rings to seat in properly, synthetic oil is so slippery that there wasn't enough friction to allow that break in to happen. 30 years ago it was so bad (ring and cylinder wall technology) that as also mentioned, we had break in oil. Lots of detergents, it was a stable but really crude formulation that allowed that break in to happen quicker. But the cylinder wall finish is so fine today, with rings to match, that they're bedded in right off the bat.
Interestingly enough, motorcycles still have a break in period. I've had an '06 Suzuki, '08 Buell 1125R and an '11 model Suzuki and all had break in periods clearly laid out in the manual complete with oil changes at 600 miles.
If it's mentioned in the car/truck world today it's nothing more than vary your speed for the first 1500 miles or something along those lines.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10