Mineral Oil as Hyd Fluid

Phil and Judy Rose pjrose at frontiernet.net
Tue Jul 29 12:05:58 EDT 2003

At 10:40 PM -0600 7/28/03, Steve Sherman wrote:
>As a tangent to my other post about my hyd fluid leak, I am curious to
>get the lit's opinion on the emergency use of mineral oil (the type you
>buy in the drug store) as a substitute for the proper stuff.  I did not
>need it, but did pick up a bottle at a drug store in case the fluid loss
>became severe and I needed it to get home.
>What's the deal with the drug store mineral oil?  Is it safe to put it
>into the hyd system?  Will it eat up seals, ruin the other fluid, etc?
>Be good to know if this stuff can be used in emergencies...

Interesting question, considering that mere (hah) substitution of
Pentosin-7 in place of -11S generates much debate on these lists.

If you (or I) happen to be caught out in the middle of nowhere with
no hydr. fluid available except from the drugstore, and if what stood
in the way of returning to civilization was simply a few ounces of
hydraulic fluid, I might consider taking the risk of adding some of
that drugstore mineral oil in order to do a limited amount of
driving. However--assuming the drugstore stuff would even have a
fairly similar viscosity to Pentosin (note that USP mineral oils can
range widely in viscosity)--it is very likely to have a _much_ higher
"pour point"--a bad thing--which could render it unusable in winter
(i.e., for cold starting). Also the drugstore stuff's "flash point"
will likely be significantly lower than Pentosin, so more risk of
combustion. Bottom line (IMHO) seems to be--if you can avoid using
very much of it (a few ounces at most), avoid temperature extremes,
avoid bad luck and avoid extensive driving--you and your hydraulic
system might survive the experiment. But why (experiment)? Why not
pick up a liter of Pentosin to use in an emergency you can well
anticipate--and eliminate all the doubt, uncertainty and risk of
damaging many hundred $$$ of car parts (to say nothing of the car

As to ruining the seals--possibly--although the drugstore stuff is
likely to be pretty compatible (chemically); it could conceivably
cause the seals to swell more than Pentosin does (and for all we know
it could also cause the seals to shrink). As to "ruin the other
fluid"--assuming Pentosin properties are optimized for this system,
replacing a significant proportion will certainly leave you with a
"sub-optimal" fluid. Does that "ruin" it? Yes, IMHO. Although if you
live/drive only in moderate temps, you might get away with it for
quite some time.

*  Phil & Judy Rose           Rochester, NY  *
*        mailto:pjrose at frontiernet.net       *

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