Here's a copy and paste that sheds light on the subject. I've highlighted pertinent info...
Most oil filters have filter elements made of paper and are effective down to about 40 microns. Below left is a paper element expanded 50 times. In this picture, a 30 micron particle is about this big: o
Paper element
--->> 
vs. composite element
--->> 
Above right is a synthetic element expanded 50 times.
The advanced filters have composite elements made of paper, cellulose, and fiberglass, and are effective down to 15 microns or less. Typically these advanced filters also have more surface area on their elements, and therefore more capacity. To put this into perspective, 25 microns is about 1 thousandth of an inch. In your motor, most parts like pistons, bearings, and bushings are set up with a clearance of 1 thousandth of an inch, so to the moving lubricated parts a 25 micron particle is as big as the oil film, and will scratch both surfaces.
We don't want particulate in our engines 25 microns or bigger. Some companies claim that even particles as small as 1 to 5 microns cause premature engine wear, but I don't find the evidence on this topic to be compelling, either for or against. Anyway, you can see now that standard paper filters are marginal.
The paper filters let through about 10 to 20 times as much 25 micron stuff as the synthetic filters do, and about 5 to 10 times as much 15 micron stuff.
It's not enough to ask a company to what size particle their filter is effective. Imagine a screen door with some oil on it. Obviously something the size of a golf ball or fly is simply not getting through. However, even particles which are a tenth the size of the holes sometimes don't get through - spray your screen door with a garden hose and see what comes off. Oil filters are similar, except the holes are random in size, not perfectly regular like a screen door. So, company A says "Our filters are effective down to 7 microns." What does this mean? If "effective" means "we catch 15%," well, I'm not impressed.
You need an efficiency number along with the size number before you can really think you know something. No filter is 100% effective - this would require either very regular holes, which are currently impossible to mass produce, or very small holes on average, which would block too much oil flow.
Purolator makes filters in three qualities, standard, premium, and Pure One. Purolator states that their premium filters capture 97.8% @ 30 microns and 85.2% @ 20 microns. These numbers are typical of a normal paper element oil filter.
The Purolator Pure One filters capture 99.8% @ 30 microns and 99.2% @ 20 microns. This means the Premium filter is letting through eleven times as many 30 micron particles as the Pure One, and eighteen times as many 20 micron particles. Clearly, the Pure One filter is doing a considerably better job of cleaning the oil than the premium filter.
The way the Pure One achieves this filtering efficiency is by combining three different types of materials in their filter: paper like everyone else to catch the big stuff, and cellulose and fiberglass fibers to fill in the "large" holes in the paper with their much finer fibers. Filters like this are now made by Purolator, Hastings (marketed as AMS), and Champion (marketed as Mobil 1 and Bosch).
Accordingly, the best oil filters are the Purolator Pure One, Mobil-1, AMSOil, and Bosch. If you use one of these filters with one of the commercial synthetic oils, IMO you have the best protection money can buy.
Champion says the Bosch is a 15 micron filter, and the Mobil-1 is a 10 micron filter but gives no efficiency numbers. AMS claims their filter is effective to "7 to 10 microns," but again without any efficiency number.
SAE tests would tend to indicate that the Purolator has a slight advantage in filtering over the other filters named here. Additionally, the Mobil-1 (M1-110) has excellent filter media but only 60 sq. in. surface area (less than a stock filter), vs. PureOne (PL14610) with 110 sq.in. The important thing is, all of these filters have performance at 30 microns which is far superior to a paper only filter, and all of these filters have performance at 20 microns which is also far superior to a paper filter. So, bottom line, these filters will clean your oil far better than a paper-only filter.
All filters have to undergo SAE (Society of Automotive Engineers) tests to prove that they meet the engine manufacturer's requirements. The SAE J806 test uses a single-pass test, checking for contaminant holding capacity, size of contaminant particles trapped, and ability to maintain clean oil. As an amendment of the J806 test, the multi-pass test also looks for filter life in hours, contaminant capacity in grams, and efficiency based on weight. The efficiency of the filter is determined only by weight through gravimetric measurement of the filtered test liquid.
Typical numbers for paper filter elements are 85% (single pass) and 80% (multi-pass).
A new test, the SAE J1858, provides both particle counting and gravimetric measurement to measure filter capacity and efficiency. Actual counts of contaminant particles by size are obtained every 10 minutes, both upstream (before the filter) and downstream (after the filter), for evaluation. From this data filtration ratio and efficiency for each contaminant particle size can be determined as well as dust capacity and pressure loss as a function of time.
Typical numbers for paper element filters are 40% at 10 microns, 60% at 20 microns, 93% at 30 microns, and 97% at 40 microns. This means a paper filter passes about 25 times as many 30 micron particles as a Pure One. I would love to see these numbers for the various available filters, but no one seems to be talking.
Here are some other links to check:
http://www.oilfilterstudy.com/
http://minimopar.knizefamily.net/oil...study-faq.html
http://minimopar.knizefamily.net/oilfilterstudy.html