All A8 4.2L engine have plastic pipe for their oil coolers, and these plastic pipe get brittle and break over time, especially if the engine is running hot or over heat too many times. I’d replaced both 4.2L 32V and 4.2L 40V on the D2, but not on the D3, it’s a little harder to remove due to different layout of coolant pipes. The dealer would charge $2000-3000 for the job replacing this $25 plastic pipe. If the car get to 100k miles and it’s time to replace timing belt, if it’s feasible, replace this pipe at the same time, it may be a little easier, but not much.
Here are 2 success stories on replacing oil cooler pipe on 4.2L D3
Some people like to replace their plastic pipes with aluminum ones, I don’t think it’s a good idea, yes, the pipe will not break, but the rubber o rings may get brittle faster due to aluminum pipe heat.