Harley-Davidson motorcycles typically require the use of DOT 4 brake fluid. DOT 4 is a common type of brake fluid that meets the necessary specifications for Harley-Davidson motorcycles. It is important to check the owner's manual or consult with a certified Harley-Davidson technician to confirm the specific brake fluid recommendation for your particular model and year, as requirements can vary slightly. Additionally, always ensure that the brake fluid you choose meets or exceeds the recommended specifications to maintain proper braking performance and safety.