Thats true but if i remember right they changed the tappets in 1998 due to them sticking with the thin oil.
They were replaced by conventional tappets with shims to stop this so you should be able to run it on thicker oil.
Better to get it checked out though mate at your local stockist.
the only oil you should use is 5w 30 and nothing else, if you use different oil then your tappids will start to make noise, if however you nhave put a different grade of oil in flush the system out and pour new oil in and change the oil filter to...