Most oil additives are basicaly nothing more than an additive package that your oil already has (not everything is "more is better") mixed with a heavier oil base that makes it appear "stickier". The problem is that the additives and cheap base oil product break down with high heat and become "gummy" which is inherently bad for engines with small oil passages, especially since most people add the crap to their oil and go WAY longer between oil changes.
Oil additives are especially bad for turbo engines, which tend to be much more abusive to oil and its additives. Big diesels (industrial and over the road like tractor trailers) don't have as small of oil passages, espcially in and around the turbo.
Turbo engines are very hard on oil. Running oil through a small engine turbo spinning at 60,000 to 100,000 rpm is the equivalent of taking a blow torch to small amounts of the oil. And this is done continuously. Most turbos are very small babbet bearings supporting a small shaft (around .25") driven by exhaust gas at ridiculous rpms. Again why I feel so strongly about my positive experiences with T6 after abusing the living hell out of my turbo Subaru wagon.
Excellent read about oil additives and an independent paboratory test of them:
http://cdn2.hubspot.net/hub/68152/f...bell_performance_xtra_lube_four_ball_0709.pdf