Sometimes a supplier will give you a thin 8mm seal to do the job when it needs a seal that is about 15mm thick. Most of the yokes are the same spline and OD so it shouldn't be a drama unless there is a problem with the yoke itself. Or casing. Was the alloy damaged when removing the old seal? Is the oil definately coming from the seal area or from the centre of the yoke. they have a welsh plug that sometimes can leak or even get damaged.