don't think 508 products are even on the shelves here yet outside the dealer.
Can be ordered on europaparts.com , fcpeuro.com , ecstuning.com , ebay.com , blauparts.com , I'm sure I've missed some..... You can often get an oil filter and new drain plug packaged with the oil too. Should be able to beat dealer prices! Castrol Edge Professional LL IV FE 0W-20 and LiquiMoly, Ravenol, Motul, Total Quartz, available here in the states.
rmeuropean.com has Motul, LiquiMoly, and Total Quartz brands of 508 oil. Looks like dealerships use Castrol Edge 508 typically though.https://www.rmeuropean.com/Make/PartNumberSearch.aspx?pnsearch=0w-20
.... and gone straight to 508. I'll stick to 504 in our Q5
That would give you a small decrease in wear. Audi-VW probably looks at the small extra wear hit using the thinner stuff and says its acceptable. Something like this:
... which is around 10% or so extra wear, although boundary additives in the VW 508 thin oils might cut that to 5% or so. That means an engine that wears its rings out in 300,000 miles would, worse case, wear out in 270,000 miles instead. No biggie.