What Is Viscous Oil
It is a viscous liquid silicon oil that is used in the vcu to engage the rear differential when required.
The quantity required for refurbishing a Freelander VCU is approximately 150ml and the grade of oil that needs to be used is 100,000 cst.
The vcu or viscous coupling can be found connected between the front and rear propshaft of the car and is affixed to the under chassis by to end bearing mounts.
The vcu you should be replaced with a new unit or be refurbished every 100,000 kilometres or 60,000 miles.