Break-in oil is designed for a very specific stage in an engine's life: the first hours of operation after a rebuild or when a new engine is being seated. The best regular motor oil is meant for long-term use, with additives that reduce wear, clean internal parts, and protect engine components over thousands of miles. Break-in oils often use conventional base oils and do without friction modifiers that can make an oil "too slippery" for the seating process. They also usually contain elevated levels of zinc and phosphorus, commonly referred to as ZDDP, which provide anti-wear protection during the high-stress early moments of engine operation. In a mature engine, that kind of controlled friction would be undesirable, but during break-in it's what helps the engine reach proper compression and sealing.