The best additive to help with transmission shudder is a high-quality friction modifier. Friction modifiers are specially formulated lubricants that reduce metal-to-metal contact in the transmission, reducing wear and improving shifting performance. These additives work by decreasing the friction between moving parts, allowing them to move more smoothly and reducing vibration or shuddering. Friction modifiers also improve fuel economy, as less energy is required for gears to shift when there is reduced resistance between them. When choosing an appropriate friction modifier for your vehicle's transmission, make sure it meets manufacturer specifications and always follow the instructions on the packaging before use.