Oil change intervals.

[ZR]

I know I know, beaten to death but here goes another one. Pics are from an '02 Chevy Trailblazer sporting a straight 6. Currently mileage is approaching 550,000kms, been run on 10w30 full syn since new, oil n filter changes every 5000km's. Engine has never been torn down, uses less than a ltr between oil changes and hauls 1500+ pounds of tools around daily. Cover was only off to replace gasket.
He's seeing similar service life out of both diffs, transfer case and transmission. No work on them beyond one front axle seal, yes all receive regular oil changes as well.
Decided to post after speaking with another club member that works in a brand X dealership. Conversation drifted over to the ever increasing number of sludged engines he's working on where the suggested interval is 12,000kms. Seeing pretty much the same thing here. For sure if you don't keep your rides till they reach any kind of serious km's it'll more likely than not be fine, keep it longer and the money saved doing less oil changes may very well bite you in the butt.
(what may look like sludge in the pics is actually just the color of the oil itself, was due for a change when it came in).