How were they maintained? How big of a load did they haul? Gas engines don't last long under hard use no matter the oil or filter. Years ago the company I worked for had a gas engine class 8 type 10 wheel with a roll back bed to haul forklifts, every three years I would replace the pos engines the company would buy. Finally they bought a truck with a 855 ci cummins engine in it and never had to replace the engine for 20 years.
Typically they all got somewhere between regular and severe maintenance using mostly OEM filters.
Most family's and guys have half tons, or suburban type deals.
Most towed from 5-7K 25 ish weekends a year between summer posts and winter sports through the scorching desert in the summer.
So cals most popular boating destination is actually the colorado river and associated chain of lakes which are a pretty brutal tow
The combined heat, and hills is really hard on NA trucks requiring lots of RPM for extended times.
The top ends go first with all the Rpm under load taking a toll on the valve train and it starts passing air.
Its under these circumstances with the little stuff you do to keep things cool, and clean internally start to add up.
the really hard use stuff are the marine engines - cooling isnt a problem with the lake but continuous unending duty cycle from the minute you start going to you go back to idle is even worse with the Mercury 500 platforms lasting about 250 hours on eth top and 500-700 on the bottom.
By keeping the oil and everything squeaky clean you can often reuse the crank at rebuild time.
