Well, the manufacturer will tell you its a lifetime fill.
At the same time, the manufacturer will tell you to only use a fluid that meets their brand new spec, for a much higher quality fluid to withstand the higher temps, pressures and stresses of their new power steering system.
And notice, the sales of rebuilt and new P.S. parts is doing very well.
So I conclude, changing P.S. fluid can be very beneficial, it can't hurt, unless you screw up badly. Pumps and Valves cause an effect called "sheering" on their working fluids, the forces and mechanics of these thing actually "sheer" the molecues of the fluid and breaks them up, once the molecue is sheered its doesn't act the same as it did before. In short, fluid does wear out. And you need a better fluid for the stress's of a more demanding system, logically its sheering the fluid even more, thus the need for the better fluid.
Personally, I do it every 30k-33k miles on my other vehicles, I have never had to replace a single P.S. part, and I most of my vehicles go out to 230k miles before replacement. That might be excessive, 50k miles might be a good figure.
Replacing PS fluid more than once a year would be Excessive IMO. Every other year would probably be good.
My Neon R/T, spec MS5931, the first change at 30k miles, the factory fill came out looking horrible, it looked fungus had started to grow in it. I used Redline Synthetic PS fluid (meets or exceeds MS5931) and every fluid change after that, I could NOT notice any real sign of degradation. My mini-van called for MS9602 (ATF+4) and I could never find any sign of degradation, either factory fill or later fill with aftermarket ATF+4.
My 2010 Commander, I bought it used, it was a rental in upstate NY and shows signs of lots of salt exposure. At 30k mi, I changed the fluid, and it came out black and very cloudy. The fresh MS10838, is as clear as water. I've put 5k mi on it since the change and it still looks clear as water. I can NOT find any aftermarket ps fluid that specifically state it meets or exceeds chrysler spec MS10838, so I use the dealer fluid. Perhaps my Comanders original factory fill fluid was contaminated by all the salt exposure, who knows, but comparing the fluid coming out to the fluid going in, I was glad I changed it.
Basically, that has been my experience, at a 30k interval for changes, there are times I found fluid looking like it was overdue for a change and I have found fluid that looked like it was perfectly fine.
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