seeing that the clutch is only a year old,and you have had a slight amount of slipping before the slave was replaced,the first thing I would be looking at would be the adjustment of the rod on the back of the master cylinder.Make sure as John mentioned that you have at least 1/2" of freeplay at the pedal,I think the spec is 14mm,if you don't then adjust the rod to obtain the correct freeplay before you go looking any further.And normally,any oil on the clutch causes the clutch to shudder and not to just slip.

Wayne