What could cause difficulty in shifting gears after changing the manual transmission fluid?

Using the incorrect transmission fluid can significantly impact gear shifting performance in a manual transmission. Each type of manual transmission fluid is designed with specific viscosity and additive properties to ensure proper lubrication, heat resistance, and friction characteristics. If the wrong fluid is used, it may not properly lubricate the transmission components, leading to increased friction, difficulty in shifting gears, or even transmission damage over time.

The other factors that can contribute to shifting difficulties, while valid reasons for potential issues, are not as directly related to the immediate effects of changing the transmission fluid itself. Overfilling the fluid can lead to foaming and aeration, which can also hinder shifting, while a malfunctioning clutch or problems with the shifter linkage can result in shifting issues, but they are not directly linked to the fluid change, making the use of incorrect transmission fluid the more likely immediate cause in this context.