Tom,
I agree!... They make us philosophical - or at least thinking about solutions, problems, "how-too-do's" and the like everytime we are stock due to other life-living things like being bound to the toilet, in bath, waiting for dinner, having s.. ??.. Ehh.. or is it just me??.. (couldn't help it.. Hopefully my humor is ok..)

Anyway, I can't say that I'm an expert, but I think like this: The fluid and the rubber "live" together and therefor philosophicaly got to also "work" together! -the fluid has a job: -to transport the force/pressure from the pedal to the clucth, the rubber sealing at the piston has a job: -to seal the piston in the master/slavecylinder so that full pressure gets transported..

If the fluid is "thinner" than the tolerence the rubberseal gives - there is a problem - if the rubber doesn't stay "sharp" at doing it's job, the fluid again wins..

Anyway, both components are chemical, and todays rubber is often filled with carbon (to make it cheaper) - silicone is also chemistry - and a "modern" invention - What I'm trying to say is, that the chemistry of them modern components both are filled with all sorts of other chemicals -to make it easy to fabricate, to not eat the container, to make more money and so on..

In good old days, rubber came from the threes, mixed with some real oil-components from Mother Nature made it possible to make a rubberseal for the master and slavecylinder in a Volvo.. -brakefluid where again a real natural oil-product and therefore the fluid and the rubberseal "knew" each other and the basic partnership was ground-sure... No big fights there!..
Today everything is so filled with chemical-components that hate each other, that you don't really know what war is going on, until one of them "show" themselfes as a winner...

Always buy original spareparts - the specications for the part is met! -and so are the rawmaterials!

According to Volvo's own servicemanual, the fluid must be brakefluid SAE 70 R3! (I don't know if you understand that SAE-code..)

Translation from servicemanual:

"...hereby ONLY brakefluid that fullfill the terms for SAE 70 R3 must be used.."

Did I got too philosophical?..

Cheers!
/Soren