Why should it be difficult engaging first and second gears of a Nissan Almera 1995 model after a gearbox oil change?

Difficulty engaging first and second gears in a 1995 Nissan Almera after a gearbox oil change could be due to several factors. If the new gearbox oil is not the correct type or viscosity, it may not provide adequate lubrication or may not allow the synchronizers to function properly. Additionally, air pockets or contaminants in the system from the oil change can affect gear engagement, and improper adjustment of the clutch or linkage could also contribute to the issue. It's important to ensure that the correct oil is used and that the system is properly bled and adjusted.