More than the oil circulating as it comes to temp, is the actual clearances inside the engine are maximized for running at a specific temperature. The tolerances are different in a cold engine which increases the amount of wear and friction on moving parts. Tolerances are designed to be within a certain range at operating temp. Each type of metal has a different expansion rate, each tolerance is measured to account for a certain amount of expansion. so when the engine is up to temperature all tolerances are within spec. Oil flow is important however, but the biggest reason you shouldn't rev the engine when cold is because of the incorrect tolerances at that temp. "