BMW Repair: power steering fluid, occasional mistake, german manufacturers

Question
My wife mistakingly added regular power steering fluid(prestone)to our 1997 740i what damage will the cause and should I get it flushed and replaced asap?

Answer
I would flush it.

There are differences in PS fluids, and BMW (as well as the other German manufacturers) are quite specific about what to use.

(It's not a bad idea to flush/refill the PS system occassionally, so I wouldn't write this off as a total loss.  Having a wife who knows enough to add fluid is well worth an occasional mistake!)