You mentioned the specs look good. Where did you see the specs?
Your practice of changing filters and replenishing the additives with makeup oil is exactly what I do with Amsoil. An important step though is oil analysis. You need to verify that the oil is indeed holding up and that some other problem with your car, like pollution control malfunction, is not depleting the additives faster than you think.
I have 20000 miles on my Amsoil in my 200,000 mile BMW right now and I am going on to 25000. I have my oil analyzed every 6000 or so and have just done the 18000 mile results.
I am under the impression Neo is $8.00 per quart or so? I buy my 25000 mile Amsoil for about $4.50/qt and can show any readers how to buy it this inexpensively as well.