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The rear, black plastic diffuser on my RS6 is looking a bit grey.

What do you recommend to bring it back to a matt black finish

Autoglym bumper care is pretty good

Aerospace 303 is pretty good, but streaks a bit when it gets wet - its also stupidly expensive.
However, think I am joking if you like, but peanut butter works pretty well if its caused by polish residue! I hear you can get similar results with groundnut or peanut oil too, which would be better still as its a bit of a nightmare to shift.
I did the plastic handguards on my bike which are matt black plastic but had become marked by polish - they got that white stuff ingrained in them that's hard to shift and makes them look grey. They came up like new by rubbing peanut butter into them, left it five minutes and wiped it off again with a microfibre cloth. Seriously give it a shot, try a small area if you want.
Other options that I have used with success are Poorboys Natural Look which is pretty decent stuff, Armourall - but has the same problems as 303 with the streaking (both are products I would limit to under bonnet plastics or interiors now), Meguiars Endurance Tyre Gel brought up the black plastics a treat too but use sparingly and wipe off for a satin finish.

If you are going to a show and want it to look REALLY good, use baby oil!