Hydraulic fluid can make a mess if it gets on your car's paint job. It is oily and leaves a slimy residue on the surface of the car. Hydraulic fluid doesn't cause damage to the car's paint job, but it should be removed since it allows dirt and dust to stick to your car much more easily. You can remove all of the hydraulic fluid from your car's paint without causing harm to the paint by using harsh chemicals.
Use car wash soap to clean the area where the hydraulic fluid is. Apply the soap directly to the car's surface. Don't mix the soap in a bucket with water. This keeps the concentration of soap stronger. Rinse the area thoroughly.
Wash the entire car with car wash soap to make sure the hydraulic fluid hasn't got on any other areas of the car's paint. Dry the car and look at the beading of water on surface while rinsing to make sure the fluid is gone. If any fluid remains, the water will bead off as if the car has just been waxed. If so, wash that area again.
Wipe the area with wax and grease remover and a towel to make sure all last remaining spots of the fluid are gone. Wax and grease remover doesn't harm the paint in any way and evaporates, so it doesn't need to be cleaned off.
Never use a product like WD-40 or similar degreasing products directly on paint. These products are damaging to paint and will ruin any future paint work done to your car.