What fluid would most likely go in a rear differential?

The fluid that is most suitable for a rear differential is a multi-viscosity gear oil, specifically one like 75W. This type of oil is designed to perform well under the varying temperatures and conditions that a differential experiences. The “75” indicates the oil's viscosity rating at lower temperatures, ensuring that it flows properly to lubricate and protect the gears immediately upon startup, while the “W” stands for winter, showing its effectiveness in colder conditions.

The second part of the rating, such as the “90” that would often follow (making it 75W-90) in formulations, indicates the oil's performance at higher temperatures, crucial for maintaining lubrication as the vehicle operates. Differential gears experience significant pressure and temperature, making it essential that the oil can maintain its viscosity and performance throughout a range of operating conditions.

Other options like 0W, 5W, or 10W are generally designed for engine oils and do not possess the appropriate properties needed for differential applications, as they are not geared toward handling the specific thermal and mechanical stresses found in a rear differential setting. Thus, a viscosity of 75W is specifically tailored to ensure the effective operation and longevity of the differential components.