This is a 1 litre bottle of hydraulic fluid for the central reservoir. Used in the central hydraulic systems on the car.
The Rolls-Royce and Bentley mineral oil system went all the way up through to the end of the Crewe era, before Bentley was bought by Volkswagen and Rolls-Royce was bought by BMW, then when the Bentley Arnage and Rolls-Royce Silver Seraph were introduced the BMW based system was used, which uses this central hydraulic system fluid.
A pump drives the high pressure fluid to the suspension, steering and brakes. It is effectively power steering fluid but it is used in the suspension and brakes as well. This fluid applies pressure to a brake booster which is then attached to a normal type of master cylinder, which when the brakes are engaged, pumps DOT4 brake fluid to the calipers. This system therefore uses a regular brake fluid system with a booster and a high pressure booster attached to it which provides extra braking force.