A majority of oils available in the market is suitable for all seasons, so they can be used both in Summer and in Winter. However, it should be remembered that after the Summer the used oil may hinder the engine start and may worsen its protecting properties of the engine against wear and tear. This is because of possible thickening of ageing oil caused by thermal loads in Summer. Possible changes will be recognizable in particular in the Winter period. It is safer to change the oil before the Winter start, especially in case of Diesel engines.