I used to not worry about it because the increase in add oil largely offset the increase in contamination. But when you see turbo direct injection engines with 300,000 mi that go 8,000 mi without needing add oil it's hard to believe they don't have increased blowby and contamination and are not doing a job on the oil when held to the same OCI as when younger.
I understand exactly what you were asking in your original question.
As a engine wears it stands to reason that the oil needs to be changed more often. At what miles does it start to need to be changed at 8k instead of 10k?
Only a oil sample could tell you that.
Now lets assume from day one a brand new Civic you use a top quality full synthetic oil and high quality oil filter and are religious about oil changes, never more than a couple hundred miles over, you use the same oil and filter forever.
Grandpa drives the car, in theory 10k oil changes will be good forever and no adjustment will be needed.
Teenage boy drives the car hot rods the hell outta it, oil changes will need to be adjusted.
Part of the reason for oil changes is to protect the engine, in theory as long as you change the oil BEFORE it goes bad no extra wear should happen and 10k will be fine for as long as you want.
Frequently switch brands of oil and filter without doing a oil sample and sticking to 10k oil changes, you are risking leaving worn out oil in the engine and increasing wear.
Now, in practice, I have been a truck driver for 30 years and a fleet manager for the last 18 months.
Trucking companies do oil samples to determine the optimal oil change interval, they average the whole fleet and leave a little wiggle room.
They stick to one motor oil and one filter brand and never switch, if they do switch they do another oil sample.
Once the oil change intervals are determined, they leave them at that for as long as they power unit is on the road. There is no “every 3 months or 3k whichever is sooner”, it is 22k regardless if it takes a month or a year. And these engines last well over 1 million miles.
I‘m using 22k as a example, some go as long as 80k, some as short as 10k, just all depends on how the unit is used.
Now, as a fleet manager, I have studied the engine that is in all of my units and what Ford recommends for oil change intervals, then at 50k I did a oil sample.
I am using a synthetic blend 15w-40 diesel oil in the Ford 7.3L Godzilla gasoline V8.
Oil sample said I could easily go another 1,000 miles on a oil change, I am doing 10k and I will stick with that until the units are scraped.
The oil change monitor on my units comes on anywhere from 7-8,300 miles. My guys ignore it, I do 10,000 mile oil changes. Period. The end. I am religious about it. I am using the same oil and filter for the foreseeable future.
Fluids are checked once a week, I top off as needed.
