When i got my car with 100k km, the inside of the engine was already clean like new thanks to being serviced at the dealer every 10,000 to 15,000 km with 5W40. In my 100k km of ownership i changed to oil approximately every 6,000 km with whatever was on sale, mainly synthetic or semi synthetic euro xW40.
I think it cleaned it even further since the filters now come out looking like new and they used to catch fine chunks of carbon. I felt it was stupid to change filters that didn't have anything in them and left them in the engine for 2 OCIs (roughly 12,000 km).
Now, even with a longer OCI of 10,000 to 13,000 km the filter is clean, not even brittle or anything and the oil still has a nice color. Maybe this has to do with my driving which is 60% highway, 40% country roads and no city driving at all, all on E85.
How would you feel about keeping the same filter for 2 OCIs for around 20,000 to 25,000 km? I know it's like putting your dirty socks back on after taking a shower, not that a filter costs much but it looks like it doesn't accomplish anything at all. I'm interested about your opinion.