Darned good point! Diesel oil soots up rather quickly and I imagine would become very "goopy" come the 7,000 mi. mark? Gas engines are a different breed and circumstance and with synthetic would probably be "ok" that far (note the "apostrophes..."). Other than my Solera MB I also own three gas engine auto's and rarely let them go past the 3,000 mile mark...usually 2,000...even with Walmart synth.
Just had to have the valve cover gasket and spark plug-well seals changed on my 95' Accord (200,000 mi. +...dunno as the odometer stopped working long ago) and the tech went out of his way to comment on how CLEAN the engine interior looked
around the rockers, etc. I have NEVER(EVER!) had an engine burn oil on me and have driven many a kind (including my '84 Sub. diesel) over the 200,000 mile mark w/o that problem. Other petty problems yes, but never that. Of course I always change the oil & filter myself( since I took my first votech auto shop class @ 16 just to learn how to $ave) and it has paid off ever since.
So, yes, not going past the 7,000 mile mark or better yet, and\or the one-yr.
rule is a very sound one and simply cheap insurance against such an expensive engine. You take care of "it" and "it" will take care of you, right? Your brother is spot on...PB