don't make a $1,600 mistake,I just made a $1,000 mistake to have my clutch replaced only to find out it tranny not the clutch ,I think in most cases bad clutch only sliping,also it's unlikely the oil is the problem if shifting has been fine in the last 60k miles,you may want to wait till it get worest(sometime its hard to see untill something is broken)than takr it to a tranny shop, they should be able to put a new clutch in without labor cost to you when do the tranny.