@@GarageWorx I can’t remember the last time I was told that oil viscosity was meant to meet fuel efficiency. Oil’s primary purpose is to lubricant and reduce friction of the engines moving parts and seals. The selection of oil viscosity or oil type is decided based on engineering principles and data during the design of the engine. The one they go with is the most neutral among the group based on operating temperature and the ware or condition of the oil over a certain period of time. Usually 3-6 months and a pre-predicted mileage. So the oil type is not necessarily going to be the same depending on the climate and season of the area you live in and how hard you drive the vehicle. Of course high mileage and engine ware over time will affect the mileage due to compression loss and un maintained replaceable parts. But having the right oil to keep it lubricated and changing it at the correct intervals is key to its longevity.
