The rate at which bits of metal are scuffed off the moving parts, especially the pistons and con-rod bearings, is much greater when the engine has just started from cold, than when the engine is at normal running temperature.
When the engine is stationary, the oil drains out of the oil-ways. Synthetic oil is good because the coating of oil left on the cold metal surfaces is thicker, and stickier, than oil distilled from crude.
When starting, the problem is to get the cold sluggish oil back into the oil ways and flowing vigorously under pressure into the bearings, so as to minimize the number of seconds during which the oil film is inadequate or missing and the bearings are scraping metal-to-metal.
When starting the engine from cold, even on a hot day, there is still a period of a few seconds before the oil in the oil-ways gets up to full flow. In the old days, the advice used to be to drive off asap after starting, on the theory that putting the engine under load warms the oil quicker. That's not so true now, with computer control of engine temperature during warm-up. Leaving a modern car to idle for a few minutes, under no load, is probably the best thing to minimize start-up wear. Never rev the engine hard, immediately after starting -- summer or winter.
The major benefit of using a block-heater is to warm the coolant so the car-heater is ready to warm the occupants, in very cold weather. Reducing the time it takes to get the oil in the big-end bearings up to full flow would be beneficial if it could be done, but a block heater has only a marginal effect on that.
