stewie

I did the wirral egg run yesterday which was absolutley brill except for a gear problem, basically I could get it in to first gear but it then wouldnt release in to second or vice versa, it meant spending a good part of the 90 min run holding the clutch in cos it wouldnt go into neutral either, once running though and a bit of speed built up it was fine, any ideas whats causing it on me 2001 curvey ? is it normal or is the gear selector shagged ? I topped the oil up recently and might have used a 15/40 instead of a 10/40 could that be a reason ?
Thanks guys