In brief: viscosity, only what the carmaker recommends

Engine oil viscosity must match the carmaker’s recommendations! By raising or lowering the viscosity, you force the engine to run outside its design conditions, and that means less power, higher oil consumption and an overhaul that much sooner.

As a rule, the manufacturer approves not one viscosity for an engine but a whole range, for example 5W-30 and 5W-40, and you should only choose within it. An oil that is too thin won’t hold the oil film in heavily loaded components (more on this in viscosity too low), while with an oil that is too thick the engine runs hot all the time (viscosity too high). There is no such thing as a “sports” or a “city” viscosity, and high mileage is no reason to step outside the approved range (see popular misconceptions about viscosity). Which range is approved for your engine is written in the service book or the owner’s manual, so look there, not at the advertising on the can.