Blackstone Labs I've used them, it's $20. 00 bucks and I think $10. 00 more for the TBN results. They will send you a free sample kit on request, then send you two more sample kits automatically after they do your first sample.
My very first sample was lost in the mail. Blackstone never received it, 2nd one I had the email report results pretty quick,with a hard copy soon to follow.
I didn't opt for the TBN on my report, I believe that's for the ones that want to go longer on oil change intervals. I normally change at 5K intervals, but Blackstone said on last report try 6500 miles.
Tony