Generally winter tends to be harder on the oil and on the engine, thick oil in cold start ups means more engine wear, also cold running means that a small fraction of the fuel is not burnt and ends up in the oil and dilutes it , since modern cars virtually no longer overheat under normal driving conditions, you could say that warm weather is better for the oil and for the engine, hotter oil at startup means the oil will flow better reducing wear, and also the engine will take less time to warm up which means less emissions, less carbon deposits, less raw fuel going into the oil etc...