Thanks everyone for all the information shared. Really useful! I am confused now. I am planning to do my PS fluid change (964 1990). I will do the dilution method (aka turkey blaster) . The TSB clearly states that Dexton IID and Pentosin CHF 11s can be mixed. Where did you get that these fluids cannot be mixed?

Also some suggested that the dilution method is a waste of time and I don’t get why. I understand that this is clearly not the best method but still a good method to consider. When we change engine oil in our P-cars there is always an amount of oil that cannot be extracted. I thought that the same principle applies here. But please weigh in and let me know what your thoughts are. Just trying to do what’s best for the longevity and the quality of our cars. Thanks in advance !

Depends on your situation
do you know what's in the car now and how old it is?
i took the system apart to fix rack leaks so had the benefit of starting with a clean system
i had to deal with some corrosion so water ingress and other rubber detritus are things to consider

the dilution method only works if done to a regular routine
if you don't know, drain

Thanks IainM for your reply - to be honest I have all service records from day one (lucky guy!) so I will check again...

That being said and regardless of what fluid is in there now, what’s the risk you see in doing the dilution method here several times until I see that the PS fluid comes new? If not filled with Dexton II chances are that it has already pentosin....original owner owned it for 27 years and was very meticulous with maintenance...but this is speculation...I hope nothing weird was added in the past. The only reason I am engaging on this project is for peace of mind and overall longevity of the components. There is nothing wrong with it now. So again, and back to my original question...assuming that the PS fluid has never been changed, what’s the issue with the dilution method? It will take much longer to take old fluid and contamination out of the system but at the end of the day I can significantly improve where I am now without the need to disconnect hoses etc...which can also lead to additional risks of breaking old hoses, rubber etc ...as always your comments are greatly appreciated!! Thanks!

In my case, I got some water in the PS system and it sunk to lowest point in the system; the rack. The resulting corrosion shredded the seals and started the leak
-not sure the dilution method would have prevented that.
for me, peace of mind has to have a degree of certainty to justify the effort but it's very subjective. Hopefully your records will show some prior work
BTW what's the recommended service periods for fluid, belts? Or is it just inspect until failure?

Just wanted to report that today I finally changed the power steering fluid in my 1990 964 C4. As mentioned in an earlier post I followed the dilution method and used Pentosin CHF11S. I ended up using two liters until the fluid came green. As discussed previously, this method does not guarantee that all contaminants are out of your system. In my case and after two cans of Pentosin the fluid was good enough for me....green and with very minimal darkness or black residue...

I know I did this against the crowd as most people recommends never to mix original ATF Dexton II D and other fluids. In this case though I trusted TSB issued in 1996. That bulletin clearly states that pre-1996 vehicles can use Pentosin CHF11S and that Dexton II D and Pentosin can be mixed. I’ve read all the horror stories of seals and rubber being destroyed but to be honest I could not find real examples corroborating that...I am not saying this cannot happen simply that I could not find examples corroborating that...so again, I will stick to TSB from Porsche and see what happens. So far, after the change the car was great and PS worked beautifully. I will report back in case I detect any leaks or issues in the future.

I don’t think there is a service interval period for PS fluid (I never understood why...) but to me this is a MUST in our vehicles. It was great today to see all that new fluid in the reservoir!!

Thanks everyone and please comment and share your thoughts on this topic. Just trying to bring this topic to an end hopefully ...