What should be changed transmission fluid or differential?

It depends on the specific needs of your vehicle. Transmission Fluid should be changed based on the manufacturer's recommendations, typically every 30,000 to 60,000 miles, to ensure smooth shifting and prevent overheating. Differential fluid also requires regular changes, usually every 30,000 to 50,000 miles, to maintain proper lubrication and prevent wear on the gears. Always consult your vehicle's owner manual for the best service intervals.