Q: Best motor oil for a 04 bmw 325ci ow40w or 5w40w full synthetic

The difference between these two oils is primarily the viscosity or thickness of the oil. Depending on the brand, there may be a number of other additives in play as well, however the most important component of the oil is the viscosity and ability to protect against thermal breakdown due to heat. Generally, when the weather is colder outside you will want a thinner oil to allow the oil to flow easier through the motor allowing it to properly lubricate parts. When the weather is hotter outside, you will want a thicker oil to prevent it from what is called “thermal breakdown” and to keep it layering the internal engine parts with the proper lubrication. I would recommend having an expert from Your Mechanic come to your home to perform an oil change service on your car to determine the best option for your vehicle.