“Oil” and “fluid” are pretty much synonymous when it comes to hydraulics, with fluid being more generic and less specific. After all, ground glass, mud and gravel are also fluid. Use of which term reflects your knowledge base. There are many formulations of hydraulic oil and most will have the AW designation. This is part of the additive package. There are two different base stocks used in formulation – synthetic, with an ashless zinc free package, and standard from paraffinnic base stocks. Most good standard AW hydraulic oils will carry an AGMA rating of 1 to 5. Hydraulic oils are available in viscosities of 10, 32, 46, and 68. (Personally, I use a multi-viscosity hydraulic oil ISO AW 32/46/68.) Many of the oils may be substituted for others in certain cases, others not. Many wives tales and misconceptions revolve around the use and applications of hydraulic oils.