Power Steering Issue

[o2bad455]

SYMPTOMS: We just encountered the same loss of steering assist (as here: https://xoutpost.com/bmw-sav-forums/...-suddenly.html) without any warnings or other symptoms on our '08 E83 X3 N52 M-Sport (USA Spec). The power steering (PS) simply stopped assisting after stopping for gas on the way into a congested city, which almost made us overshoot an exit ramp and then made tight curb-side parking unexpectedly challenging. There is no noticeable fluid movement in the reservoir with the engine running, but the belt's good, pulley's not wobbling, no noises or PS leaks.

CONTROLLER?: I thought it might be a steering controller fuse, but fuses 66 and 69 both seem to be fine. Is there any possibility that this could be a controller issue? If I understand correctly, although it seems not to be well documented, the steering controller supposedly actively adjusts the amount of assist. Anyone know if it adjusts to zero assist under any circumstances, even if just in a failure mode? I couldn't find any relevant fault codes using INPA, but just got ISTA working again (under an updated VBox Win10 VM, FWIW) and it just reported code "0000A1", description "MRS: No message from DSC control unit or message incorrect", and mileage "-1". Could that code be relevant? The only two other codes (for gearbox oil wear and a number-plate light) both showed actual mileages.

RESERVOIR?: I read that the PS reservoir has a non-removable filter. If it got plugged somehow, could that cause the issue? Has anyone successfully removed the internal reservior filter (I'd add an external one, of course)?

PUMP?: If not the controller or reservoir filter, I guess we'll be ordering a new PS pump. But I've truly never had a hydraulic PS failure anything like this before. It makes me wonder: Are these PS pumps' impellers made of plastic? If so, is there a metal impeller version or upgrade available? Our vehicle assembly date per the door-jamb VIN sticker is January 2008, although ISTA just identiifed it as September 2007 (I presume retrieved from a control unit, but no idea which). It looks like there were several different pump versions in a 2-3 year period, so I guess I ought to at least pull the lower shield off to take a picture before ordering.

FLUID?: I will say that the fluid had been right at the min mark when cold and about halfway to max mark when hot since we got the X3 a couple of years ago. When on a long trip this past summer, I decided to top it off to full when hot using Prestone European Power Steering Fluid, which listed "BMW" on the front label and "CHF-11S" in the fine print. After pump replacement, exactly which readily available fluids should we use or avoid?