I did hear that it is very important that you follow the oil spec that is specified for this motor ... thinking that oil barrel of 15W40 that you kept for your other tractors will be fine will cost you. Apparently the little turbo will starve and then go boom - sometimes taking other engine parts with it. But that is just what I think I heard (being a senior my hearing is not always the best) from an LS mechanic.
You can't run a Turbo-Diesel all day long at peak power then turn it off real quick to go have lunch. You can't even run them for an extended period at power and turn them off abruptly.
Those Turbos get hot. Real hot. Really, really hot. And when you cut off the air-flow that they get while running, they'll cook themselves. You need to let them idle for anywhere from 30 seconds to 5 five minutes, depending on how heat-soaked they are. Most of the time, just a few seconds if you haven't used the turbo much.
It also helps if you run something other than that old generic 15w-40 left over from your 1974 JD that's in the corner of the barn.
These new engines from LS are state-of-the-art engines. And they require more care and more knowledge than the old indirect injection diesels. Clearances are tighter, machining is far superior..... So many improvements and differences.
Would I be surprised to find there is a learning curve in these engines and there might be some things that need corrected/fixed? No, I would not.
But 7 engines in one dealer's repair shop?
I don't think so. Maybe more like, the mechanic has tried 7 times to fix the same engine. I'd buy that one.
I think the mechanic was funnin' the OP.
If 7 engines went back to a dealer, ANY dealer anywhere, the factory would shut down production until they tracked down the problem.
I've heard ducks fart underwater before.