I personally haven't used Blackstone for anything but my diesel and when a oil change means 15qts of syn the 20 bucks is less of a big deal. I have made it over 12k so far on my last change without having a dual bypass. When I use them I don't Drain all of my oil either, it's just a qt or so, then get the sample. It's amazing the stuff that they can tell you about your engine and thats why i'm gonna keep doing the 20 bucks every 4k to see if I need to change my oil.

I have a friend who has a 4.3l and he is at around 10k on his last oil change. By using Blackstone.

It's all a give and take. If your putting 2 qts in between a 3k oil change interval then the 20 bucks probably isn't as good of a deal, however a once a year test on the oil might be good to see whats going on in the engine based on your last sample.