There is a plastic blue 13mm hex drain plug on the bottom left of the radiator. You can get most of it into a clean bucket.

The oil level sensor if bad should turn the oil light on orange when you first start the car. Not sure why you would have the code and not see that light. I would just clear it and see if it comes back. There is a dipstick so I'm not really sure why there's a sensor. And of course on the newer cars there's only a sensor which is really stupid.

The ABS light is probably a wheel speed sensor.

And the brake circuit issues, I don't know, on the E36 I had that a whole bunch, and it was because the wiring harness that goes up into the trunk lid that flexes gets frayed and arcs on itself, fire hazard. I'm not sure if the brake light wiring on the e53 tailgate if they fixed that. I would think so. I haven't heard of anyone having these problems with the trunk wiring.

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'05 E53 3.0 6mt
'17 F30 340ix 6mt
'96 E36 328is, in progress