The following video shows how to reset the service reminder on a 2016 A8, so can't guarantee if it's the same, but worth checking out. The first part deals with the oil level reset, so the link starts at the main service part:
https://youtu.be/O9XlBhWnTM4?t=76
For the dipstick, oddly enough the parts catalogue shows a dipstick in the image, but doesn't give a part number. I tried a search for dipsticks for the CDSB engine, but got nothing. All the searches for the 4.2 dipstick were for the petrol version.
Something you could try, but you would need to do some tests during an oil change comparing the MMI and dipstick to see if it was anywhere near accurate. The dipstick part number for the 4.0tdi & 4.2tdi D3 engines is 057115611H.
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
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2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds