Ability of a hydraulic fluid to cover the surface over which it is distributed to a greater or lesser degree.
Hydraulic fluids with good wetting ability adhere better to the surface. As a consequence the lubricating film has better load bearing in the boundary layer area. This aspect is particularly important for seals where with poor wetting ability the lubricating film is pressed away at the sealing surfaces such that dry friction occurs.
The wetting ability of a mineral oil depends on specific polar additives that are contained for example in detergent oils (HLPD oils).