I feel like its a holdover from carburated cars or early EFI. I don't feel like it has much place in a modern engine. I'd be more worried about damaging a seal etc with it than any good it would possibly do.
Like I said, I've been cautious. Blue Devil products have decent ratings among customer reviews, but here and there you'll find cases where the user overfilled the tranny, or the seals were swollen so badly that the tranny started leaking like a sieve -- opposite the intended effect.
The engine Main Sealer and a similar formulation -- both Blue Devil products -- seem to perform more reliably. I personally attest to it. But I avoid products which remove "varnish" from the engine. It could be asking for trouble.
The Trooper and its engine are 188,000 miles and 25 years old. I'll go 3,000 miles, and never have to top up the oil. At one point, I'd go 3,000 miles and the oil would be a half quart (of five) low -- but this was apparently due to the leaky rear main seal. For the additives mentioned here, I've never used the full bottle.
I figure if you have a 25 year old vehicle with smog test results better than it had at 6 years old, it's probably worth it to change the oil every 2,500 miles even if using Full Synthetic. Baby that Sucker, I say! If an engine is going bad, "getting tired", etc. -- you would notice things gradually, like worsening smog-tests, burning oil, compression tests falling short. If none of that is happening at 188,000 miles, getting to 230,000 or beyond seems like a good bet.
I still wonder about my head gaskets. In 2009, the water pump froze, timing belt broke, radiator was damaged. We thought there might be damage to the engine, but "Nope." I was told that if damage was done to the head gaskets, it might show in a month's time, but "Nope." That was at least 30,000 miles ago. So like I said, it seems like a good bet for keeping the Trooper, paying for the peripherals and maintenance, and saving my money.
I might "try" additives, as I have with the Main Sealer or fuel-injector cleaner. The Lucas cleaner is harmless. But otherwise, why screw up a good thing?