So I have done lots of research on this and I think I have closed the loop on this. The required oil for my 2021 Chevy Silverado 1500 3.0 liter Diesel is DexosD 0W-20. So there are two oils currently listed by Dexos that are DexosD: AcDelco light duty diesel and Mobil Super 3000 XE2. The AcDelco is of course available from the dealer but as you have heard above, they are not always using the correct oil and/or it is on back stock. The Mobil Super 3000 seemed like the right choice, but I could only find it on ebay for $74/qt. I did stumble upon Mobil One ESP X2 mentioned in forums that was supposed to be DexrosD certified and I found it at SummitRacing.com for $10/qt. I went ahead and ordered some and inspected it once it arrived. Nowhere on the label does it mention it was DexrosD certified so I called Mobil One (never have before but highly recommend it, they are super helpful). Marcos (Mobil 1 CSR) explained to me that Super 3000 XE2 is only available in Canada, so that explains the rarity of finding it. He did confirm that ESP X2 (Mobil One Part # 125386) is the correct DexosD certified oil, but here is where it gets interesting. The oil I received from Summit and the one currently being sold from NAPA (yes I actually found some on a shelf locally) were labeled and branded as ESP X2 and were correct part numbers, but the branding label and color scheme (DexosD Certification) doesn't match what Mobil one has on their website. After sending pictures back and forth and confirming Part Numbers they discovered that what was being sold now was produced in 2017 in Louisiana specifically for high end European cars in the USA. Of course the oil is the same make up and it is truly DexosD Oil, but it was just bottled before DexoD certification was a thing. These come with Green caps, and the labels do not mention DexosD, the new ones are supposed to list DexosD and have orange in the branding color scheme. Both will have part # 125386 on the box of 6. These are only sold in quarts. To summarize, I was told by Mobil 1 that ESP X2 0w-20 Part # 125386 was the correct DexosD oil for my 2021 Chevy Silverado 1500 3.0 Liter Diesel. Of course your Truck your liability, so feel free to call Mobil 1 at 1 800-662-4525. Hope this helps.