I posted this in another thread, but it may be more appropiate here:
In the early seventies when I was in High School auto shop class, we were given a project to rebuild an engine. The local junkyard would drop off about 12 engines from their stock of wreaks. These were Buick, Ford, and other V8's from 60's era cars. Each group of two people were given an engine to work on, and first thing that I noticed was how much sludge was in these engines. The valve covers were totally covered with 1/4 to 3/8 of an inch of black sludge. The heads, and all internal parts of the block were covered with this stuff. All of the engines taken apart were in the same condition. Now fast forward to 1996, and I had to change the valve cover gaskets on my 1991 Ford explorer; the inside of those covers were practicaly like new with 150,000KM on the engine. There was maybe a thin film of sludge in one area that could be seen by wiping a finger down the inside surface. This is how far oils have come in 25 years. I don't think sludge is anything to worry about anymore.
Also, we did the "oil filter" study long before there was an internet. The first thing that our teacher had us to was to remove all the oil filters from the Junk engines that came in, and to cut them open and measure the length of the paper inside. He would use this information to purchase the brand of filter that was best for the coming year. I still remember the results from 1972 in anyone gives a **** . The best filter was a Mopar with 12 feet of paper, and the worst was a Sears filter with 4 feet of paper element.