The oil is a silacone and is thick enough to cause resistance between the internal parts of the fan clutch causing it to spin the fan. It can still be slowed down compaired to engine speed so it don't spin too fast at high rpm. The looser the clutch the better the mileage, the tighter the clutch the worse the milage and noise factor.
We used to pack the Nissan clutches with STP and that worked ok, but you can't go wrong with the factory stuff.
