Conventional wisdom says that a new engine should be run-in on conventional mineral oil, regardless of your intentions to use a synthetic for the long haul. The conventionally wise say that synthetic oil is too slippery and won’t let the microscopic high points properly lap themselves in, delaying the break-in process.
I say rubbish. Many modern cars, notably such high-performance marques as Porsche, Ferrari and Corvette, are factory- filled with synthetics. You can bet that somebody has determined that the break-in process will proceed normally with synthetic in the sump of these ultra-high-performance engines. And that goes for your Toyota or Jeep as well.
Nonetheless, I do prefer to use a mineral oil for break-in. It’s quite a lot cheaper, so I don’t mind changing it after 50 km and again at 1 500 km. I would not, however, change the factory-fill synthetic back to mineral for run-in. Those vehicles typically have carefully assembled engines with instructions to do the first oil change at the regular interval, which could be up to 15 000 km. In those cases, I simply change the oil early, before 1 500 km, just to be safe.
Synthetic oil is a superior product, particularly if your engine operates at the extreme ends of the temperature scale: high temperature climates, towing or racing. But like any oil, synthetic will become contaminated with atmospheric dirt, wear particles, carbon, partially burned fuel, water and acid. Eventually, even if the oil itself is performing properly, all this extra junk will manifest itself as engine wear.
Also, the first oil change invariably reveals small particles of gasket sealer, chunks of unidentified plastic, the occasional metal flakes that weren’t cleaned off before assembly, and even the odd washer or nut. It’s pretty scary. Better this junk comes out sooner rather than later.