Long gone are the days when a can of ATF was all you needed to top off a power steering reservoir. For years now, savvy technicians have known to use dedicated power steering fluids for the many models, both import and domestic, that call for them. There are still many cars on the road for which ATF is appropriate.

However, in recent years, some European automakers including Audi, Volvo and Mercedes-Benz, have specified a more advanced synthetic fluid for use in power steering systems. This fully synthetic blend is designed with selected esters and certain anti-wear characteristics to assure longevity of the various alloys used in the sophisticated power rack-and-pinion units in these vehicles.

It maintains a very stable viscosity over a particularly wide range of operating conditions, assuring proper lubricity in the extremes of high desert temperatures, as well as remaining free flowing in extreme cold, even down to -40 to -50° F. In ­addition, the advanced chemistry of these new blends allows for effective lubrication and ­protection, even with the extended fluid change ­intervals as specified by these automakers.

Mixing this new blend with previous petroleum-based fluids is not recommended. However, replacing the petrochemical fluid in older models with this new synthetic fluid is a viable upgrade, and is easily accomplished with a thorough flushing of the old fluid. Do note, however, that Honda-specific power steering fluid is recommended for Honda and Acura models.

The new synthetic fluid is often identified as CHF11S in owners’ manuals and service manuals. It is also readily identified by its distinctive green color. So while it’s always best to refer to manufacturers’ reference data, as a general rule, you’ll be safe adding “green to green.”

Note that while fluids may be described as compatible with CHF11S, some fluids are blended to exceed the standards of such fluid, sometimes by a wide margin. So, it’s best to check the specs of a given fluid, or choose a brand you trust to meet or exceed OE recommendations.

Contributed by Steve Muth, chief chemist, The Penray Companies.