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Sometimes it depends on which year Audi you have, but I would say go with the synthetic with any year that you have. ;-)
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"All 1990 - on Audi models have come filled from the factory with the synthetic CHF 11S Audi hydraulic fluid (G002000). Please Note: Never mix generic hydraulic fluid or other fluid with the CHF 7.1 or CHF 11S Audi hydraulic fluid (G002000). If you or someone else did, flush the Audi hydraulic fluid as described below." Now are you sure it is the P/S pump making the noise?
> At the moment I am experiencing the same problem with noisy cold
> start-up. I recently had to change the oil and put in generic fluid.
> Since the oil change it has been noisy. The previous post "Synthetic
> Mineral Oil" is interesting. I have never heard of this. The dip stick
> indicates top up with mineral oil. Does anyone know the desired oil to
> use. I think this may be key.