I believe that any "seal expander" that's in ATF will only cause the seal to swell about 5% and suspect that similar things in commercially available power steering fluid will do similar. No doubt, the brake fluid will do similar things.
One thing to be cognizant of is that the same fluid that goes through the power steering pump also goes through the steering gear which also has rubber o-rings/seals in it and in the valve body of the gear. You're not affecting just the seals in the power steering pump, but the whole system. Something about fixing the leaks the correct way instead of "something else" seems more cost effective to me in the long run, but your orientations might vary from mine, which I respect.
The 20W-50 motor oil can work too, but a lighter grade might more closely match that of the power steering fluid's viscosity and take less horsepower to run the pump.
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