I just had a free oil change at the dealer on the '21 Sierra. I scanned the invoice and noticed it said oil used 0W-20 Dexos 1 Gen 2. The truck specifies 5W-30. I turned the paper around and asked the service writer if this was the oil put in my truck, he said if that's what the invoice says then that's what was used, why? I said the truck calls for 5W-30. Another older service writer heard me and came over and said the techs put in whatever is on the oil cap and just input whatever oil was last used and comes up in the field on the form. If true that seems like lazy shoddy practice that would lead to inaccurate inventory.
So who do I believe? I'm tempted to pull an oil sample and have it analyzed. Not afraid of 0W-20 hurting anything short term but I don't think I want it in for a full OCI with some towing.
This is EXACTLY why I do this stuff myself.