The role that density plays in the performance of hydraulic oil is key. Density is the measurement of a substance’s mass as it relates to a known volume. Water’s density has been measured at 62.4 pounds per cubic foot, but this number varies when the temperature of water changes. Where lubricants are concerned, they tend to be less dense than water, which is why water will tend to settle to the sump’s bottom and is the first thing to be drained from the lube system when a valve is opened or a plug pulled.
In hydraulic systems, lubricants can sometimes be mixed. Whether done accidentally or not, there are consequences to mixing lubricants. Even mixing the same kind of lubricants can place a system at risk if those lubricants have different viscosities.