A manufacturer recommends a weight of oil based on the design of the engine.
Oil that is too thick may not flow quickly enough to fill the spaces between the crank journals and main bearings while the engine is running. The oil may not form a consistent lubricating film, allowing metal-to-metal contact and wear. Not only that, but the engine will waste energy pumping the thicker motor oil, reducing fuel economy. Since thicker oils don’t transfer heat as well as thinner oils, operating temperatures will increase, too, possibly leading to accelerated chemical break down and harmful sludge and deposits.
Oil that’s too thin can fail to develop a consistent lubricating film, inviting metal-to-metal contact that causes wear.