I have used it in two very different engines with great effect. I have never read the manufactures blurb, i was told it 'reconditions' rubber seal components by 'swelling' them keeping them soft and supple as they harden over time.
First engine was a 602cc air cooled flat twin, lots of miles, hard life, smoked on WOT and leaked. After filling with full synthetic and Lucas oil stabilizer no more smoke at full throttle and no more oil leaks, I was very surprised.
Second engine was a K-series BMW, horizontal 4 cylinder, renowned for lots of smoke at start up, not after the stabilizer, the difference was night and day.
I've not read if it is suitable for diesels, in a new engine it would be useless, but in a worn/high mileage engine it can do no harm