There is no viscosity or brand on the Carfax report, and if you show adequate oil change intervals in Carfax, nothing else matters for warranty oil change requirements. This has been legally tested many times.
You can supply your own oil and filter for many carfax reporting oil change companies like Valvoline and Walmart, at a discount. I change my own oil but for the still in warranty cars, I make sure I go to a carfax reporting oil change place and bring my own oil and filter.
