I note that they are particular as to the GL of the oil used,I checked my Series workshop book and it specifies Castrol EP 90,which I would presume was a GL4 oil,I also note that the modern Castrol is EPX 80w/90 which is a GL5 oil,and has been for quite a number of years,so the dilemma is does one use the modern oil, or go back to using the older oil tech oil.
cheers