I've only recently started cutting open filters.
Last year when I cut open the two filters that I used when I added a quart of ATF in (to clean) I cut open both filters to find that the filters were filthy (first one about 400 miles, next one about 700) and sludge was coming out of them in very small quantities.
The filter in the car that we had since new looks fine after 4500 miles. The filter I had in my race car (winter driving and 2 ice racing events, 2-3k miles) had some larger visible chunks on the fitler, but not absolutely terrible. Immediately after I bought my latest beater I changed the oil. 1000 miles later I decided to change the oil because the drain plug threads were stripped so I wanted to fix the threads. The oil was dirtier than the oil I had raced on and the oil that was in the other car for 4500 miles. I cut open the filter to see what crap there was. Trace amounts of sludge were seen on the filter, but none sitting in the can. Currently running an autoRX in both of the older cars to see what comes out.
