Like everything else, this is not a "one size fits all" situation. The only way to know for sure is to have a differential pressure setup on your oil filter. I did at one point on a now-sold truck and those experiences validated what I was told by engineers from several companies that the average oil filter is tossed with more than 50 percent capacity left. They weren't talking about premium, high capacity filters, either. That the industry is moving in this direction is further validation of the idea.
How do I recommend approaching this, and this is how most of you seem to have done it:
-start with an engine that has no known issues that would generate high levels of contamination, excess blowby, high wear metals, etc. (a very high miles engine with an unknown history or "abused child" would be more suspect)
-have a tight air filtration system to minimize ingress of outside contamination... the more efficient the air filter, the better
-use an oil capable of the desired interval but change it before it oxidizes and creates sludge (UOA to determine an interval)
-choose a filter with a known high capacity and of a quality construction (to me that recipe is a synthetic media, premium filter)
-if you can opt for a filter with more area, you are hedging the bet
Wobbly, your example doesn't provide much useful information. Just as with an air filter, what you see in the media is not reflective of restriction. It can be, of course. Everyone has seen examples of air filters that look like a hunk of carved sod that are obviously plugged, but we have also seen nasty looking filters that had not yet reached a level of restriction that warranted replacement. I use air filters as an example here because they are a very visual filtration example and restriction gauges are common so the restriction of the filter can be easily measured. In your case, with as many miles as are on that engine, you could very well be pushing the envelope. I'm speculating but what I see on yours looks like sludge to me (oxidative byproducts). That could come from a worn engine that is making blowby and degrading the oil more quickly. Or, perhaps the oil wasn't quite up to the OCI, or perhaps your OCI is too long for the operational situation (lots of short hops). Again, I don't know the details of the situation and am offering some speculative suggestions. Without testing the filter, there is no way to tell how restricted it is, but my experienced guess is not too much. That said, 20K miles is a good long interval and probably a good time to retire the filter. IMO, the only thing subject to debate (a waste of breath since we have no way to test restriction) is whether that filter was restricted to a high level or not.
