22SLT

There's a QR code on the driver's side pillar. You scan that and it brings up your RPO codes. But it's an incomplete list. I ran my VIN through gmpartsgiant.com which was surprisingly accurate, and then I went back to my dealership for the full build sheet to confirm my differentials. Also I didn't see it until now but this is the HD forum and I have a 1500. Sorry about that.

I have a 2022 Silverado with the same setup as the OP (SU4 front, SU8 rear). Part numbers are Canadian. After considerable research and lots of youtube: 1. SU8 gasket is reusable. GM part number 84412728. The SU8 requires AC Delco 75W-85 GL-4 fluid, GM part number 19368514. 2. SU4 gasket is one-time. GM part number 84428297. The SU4 requires ACDelco High Efficiency Gear Oil 75W-85 GL-5 fluid, GM part number 19300457. Edit: The picture of GM part number 19368514 shows GL-5 on the label, not GL-4. So I did more research. It seems the "SU8" is irrelevant for fluid purposes. What is relevant is if you have the G80 locking differential. I do have this one and the problem is it has brass and copper mechanisms. The sulfur-based additives in GL-5 will react with those metals so you need GL-5 gear oil with "non-reactive, buffered sulfur-phosphorus additives". Apparently a degree in chemical engineering would also be useful. Amsoil Severe Gear and some others use those 'non-reactive' extreme pressure additives and that is why they are compatible with the G80/SU8 combination. Also FYI: the transfer case uses Dexron VI synthetic automatic transmission fluid GM part number 88865549, but you can get any Dexron VI from Mobil/Valvoline/Amsoil. And in case you have it, the 8L90 MQE transmission (8-speed) uses Mobil 1 Synthetic LV ATF (blue label). Which is equivalent to Dexron HP. Hope this helps.

I have an SU4 front axle and an SU8 rear axle. Which I only know for sure because I matched photos of my rear differential and then went back to the dealership for my build sheet. Those RPOs do not appear when I scan the QR code on my door jamb. SU4 spec: ACDelco High Efficiency Gear Oil 75W-85 API GL-5, (GM part #19300457) SU8 spec: ACDelco 75W-85 API GL-4 (GM part #19368514) Lots of warnings about using the wrong spec fluid, yellow metals and EP additives and so on. But on the forums a lot of people say you can use Amsoil Severe Gear 75W-90, which is GL-5, no matter what differentials you have. Is that specifically because the Amsoil data bulletin states "Can also be used in axles where an API GL-4 lubricant is recommended"? When I get these changed I will get the correct spec fluid for each. But logically, it seems to me that there must be millions of functioning trucks on the roads where the owner or mechanic used 75-85 or 75-90 without looking up data sheets on the internet first.