There's typically also a calendar interval with oil changes, and most manufacturers recommend more frequent changes in eg dusty conditions.
A truck thats getting 100k change intervals will likely be doing that distance in a few months, and all on-road. Personally I only do about 10k a year in the landy. No way I'm only doing an oil change every 10 years.
The same oil in different engines/vehicles/environments will have a different lifespan, so unless you're doing testing as Rick suggested earlier its just a guess as to whether you can actually run an expensive oil for longer before changing.
I've got the truck type Isuzu twin filters (full flow + bypass) on my turbo'd 4bd1, and I change filters and oil around 10k using Castrol RX Super 15w40.
The NPR trucks with that filter setup had a 12k change interval, so I'm not panicked if circumstances push me out a few thousand km's over my normal 10k.
I'm not quite sure what you mean by a typical service costing $3-400. RX Super costs me $100 for 20L (so about $40 for 8L change), fuel and oil filters around $50 and even with an air filter as well you'd be lucky to get to $150 total.
Steve
1985 County - Isuzu 4bd1 with HX30W turbo, LT95, 255/85-16 KM2's
1988 120 with rust and potential
1999 300tdi 130 single cab - "stock as bro"
2003 D2a Td5 - the boss's daily drive
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