Synthetic oil will cause your clutch to slip,as it's slippery,and if used may not flush out of the linings,rendering the clutch slippery.I would use a motorcycle oil,or if you're like me,I like to say sensible,but others may say cheap,but I buy oil about 5 gallons at a time and use the same thing in everything.15-w-40 is a good all-round oil as I have been using the same oil about 10 years,and have seen no bad results and when I set the valve lash on my Nissan Hardbody truck,I Plastigauge the cam bearings,and have seen no wear.Also,the cylinders are very clean, and no wear ridge at the top after 185,000 miles. Oil made for diesel motors has a greater amount of ZDDP,a zinc additive for metal-to -metal friction,but is good for wet clutches. lha