[BiturboS4] Hydraulid fluids.

[HJN0316 at aol.com]

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What are biturbo S4 DIY'ers using to top up your hydraulic fluid reservoirs?

I've been using Pentosin 7.1 (mineral oil) in my '91 CQ for many years with
good results, but am taking some extra caution before I top up my '01 S4.

Both the CQ & S4 reservoirs are labeled "Use only mineral hydraulic fluid G
002000".  The S4's labels are accessed by removing the plastic shroud over the
reservoir.  Logic then says I should be able to use Pentosin 7.1 in the S4.
But sometimes the German definition of logic is not exactly the same as ours.

Someone suggested I use the Pentosin 11S (synthetic) in the S4, and to
introduce some possible confusion the 20v.org website under regular maintenance has
the write-up of using Pentosin mineral oil in the 20v (I-5) hydraulic systems.
 The author prefers the Pentosin 11S synthetic oil.  There is no mention of
first purging the original mineral oil or changing any seals.  Directions on
the both the cans of Pentosin 7.1, which calls itself mineral hydraulic oil and
the Pentosin 11S, which calls itself synthetic hydraulic fluid, are explicit
about not mixing the two.

My CQ owners manual states to use Audi hydraulic oil G 002000 or equivalent,
but doesn't mention mineral vs. synthetic.  The S4 owners manual doesn't
address this issue, seemly preferring to leave that responsibility to dealer
service.  My CQ Bentley echoes the owners manual, while my S4 Factory CD-ROM states
to use Audi hydraulic oil G 002000, again without mentioning mineral vs.
synthetic.

I don't know if mixing or interchanging mineral vs. synthetic hydraulic oils
is analogous to mixing or interchanging DOT 5 synthetic brake fluid in systems
where DOT 4 is called for.  The owners manuals warn against this, as, in my
understanding, the DOT 5 attacks braking system seals, etc, and can lead to
brake system failure.

I copied you Matt specifically, since you like me, have both a CQ and a
Biturbo S4.  Hopefully you're still keeping up with your emails.

So.........., to repeat, what are you using?

TIA

Hank