Fourtrack gearbox


Not sure if this made it on my first attempt, so here goes again. My N-reg 2.8 TDX (just brought) went to my local garage for a once-over and a levels check. They had to put 1 & half litres of oil in the gear-box! Since then, I've had some trouble getting it into 2nd and 5th when its cold. Is thia a common thing or should I be worried? Its got a bit of warranty left so should I take it back or get a report from a D dealer/RAC/AA first?
Many thanks, Clipper.