HPSF used to be a mineral oil based power steering fluid but from looking at my last batch pour it's very clear which leads me to believe it's synthetic. Certainly the Penrite stuff is advertised as synthetic and they claim it's not only compatible but imply you can safely mix it with OEM.Originally Posted by markismaximus
One site I reviewed suggested ...
"Its all about lubricating the seals in the pump, that the specific compound of the rubber seals requires that they be kept pliable."
My question: given that Penrite have probably "reverse engineered" the HPSF product, they may have used the same ingredients or very similar.
Another site suggests it has to with the requirements of the rack and pinion, the additives in HPSF lubricate the joints.
Peter
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