Auto Box Slow
 

Hi everyone
I have been experiencing a slight problem with my 2009 auto box on a 2.4diesel.
Most;y it is fine but it is now starting to take a few seconds to hook into the next gear. All very smooth, no clonks or jumps or the like, Just slow to hook in.
Is this just a case of changing the oil in the slush box or the sign of something more sinister?