I recently got a red cog of doom on the centre display of the M5, not the first time but last time it was the clutch and I have done that, soooo...
A bit of googling and I discover a common failure point on the SMG 3 box is the insulation on the small wiring loom that connects the shift control solenoids to the main harness, long story short, heat + years = warning lights!
As luck would have it there was an E60 M5 box on trademe so I bought that with the intention of putting it into my car while I give my box a good refresh, and then swap them over again. First issue, the replacement box also had the wiring loom issue so I did a practice run on that box to sort it out before it goes in the wagon.
box as it was when I got it, approx 140km on it, insulation is hard and crumbly....
Disassembled plug, unpinned one wire at a time and scraped old insulation off, then 4mm twin wall heartshrink and repin the wire, repeat as necessary....
wrapped the whole lot and replaced the loom clip, then covered the end in liquid electrical tape to keep water out....
Oil changed, oil strainer cleaned and refitted, new crush washers on all the plugs and filter...