I debate the question while driving to work and keep flip flopping regarding which season is harder on oil : Winter or Summer ? Trying to be logical , I assume that once oil comes up to engine temp then it's self regulating and either hot or cold temps should matter less . That leaves us with start up on to final engine running temp to suggest which season is harder on oil Winter start up or Summer start up - or about the same ? Hot , humid stop and go big city Summer traffic in the South or countless days of starting your vehicle up in temps below freezing ? Lastly , does your decision influence your oil brand and grade choices for Winter or Summer ? If I thought Winter was harder on oil - then I would use a better quality oil such as M1 5W30 EP as a Winter fill and save my lower tier Quaker State 5W30 for the Summer fill ... Your thoughts / experiences with the above ?