NAPA kits are $16 and available at local NAPA although may need to get them from the warehouse.
Blackstone is overpriced. $40 for the UOA with TBN. NAPA is $16 for the UOA with TBN.
Blackstone gives you comments encouraging everyone to extend their OCI. Ignore their comments
Some give you oxidation and nitration levels.
And why is it a problem for them to encourage extending the OCI, given the data? Btw, they don't always do that. This is a generalization. I've seen plenty where they did not do that and some where they suggested a shorter OCI.
I find Blackstone's comments to be informative. YMMV. I do agree that they are overpriced, though. But they do make it easy.
I am impressed with Napa's price. How is their turnaround time, typically, and who does it for them?