FWD — front engine, front wheel (FFs) — cars generally have the differential and the transmission in the same housing, referred to as a transaxle. So, the same transmission fluid lubricates both sets of gears. You must consult your car’s user manual to determine how often you need to change this transmission fluid. Using Honda as an example, experts there will tell you to change transmission fluid every 50,000 miles to keep it in pristine condition, but this number varies depending on your car’s make and if you have an automatic or manual transmission. The same applies to all cars that have engines with the same position as the drivetrain (MRs, RRs, and FFs).
As for RWD cars with the engines in front (FR), you mostly don’t need to worry about changing the differential fluid often. Car manuals often don’t even mention differential fluid (after quick looks at the Ford Mustang’s and BMW i7’s owner manuals). This differential fluid is meant to be a “lifetime fluid,” meaning you should be covered as long as you have the car without any need to change it. But if you use your RWD (SUV, truck, or car) for towing, offroading, or as a performance vehicle, consult your mechanic to inspect differential oil earlier because it’s seeing more stress than it was designed for.
