Most synthetic filters are in the 99% @ 20-25 um realm, which is about as good as it gets in the current market for FF filters. Some cellulose blended filters (P1, M1,D+, others) are in the same efficiency realm. The difference is capacity. The full syn filters are usually much higher capacity. One example I have at had is 18 grams capacity for the P1 filter that fits my Ford 5.4 and 31 grams capacity for the full syn Fram Ultra. Both filters are 99% @ 20um.
The flow thru a syn filter is also greater, even down the line as the filter loads up. Often, most often I think, a higher efficiency syn filter will outflow a lower (normal or standard) efficiency cellulose. Flow data for all filters is not easily obtained but I have spot checked the few I can and this seems to be the case.
Plugging a filter is not much of a worry these days. According to several oil filter boffins consulted, the average filter at a normal OEM OCI (7.5K) is no more than 50% loaded. Unless an engine is operated in severe conditions, neglected or has a defect, such as an air filtration problem, that is allowing outside contamination to enter the engine, the amount of wear metals and oxidation residue that is captured by the fitler is small. An engine that is operated in a good environment, e.g. full warmups of the oil, no excessive heat, etc., is unlikely to generate a lot of oxidation residue. Most modern engines use belt-drive cams and roller tappets so they generate little wear metal. There are specific exceptions to these generalities, of course.
Pursuing long term "engine health" via a premium filter is not a cost effective goal IMO because the difference in engine wear between an "average efficiency" filter (~99% @ 30-35 um) and a "high efficiency" (~99% @ 20-25 um) is not huge. Certainly, there have been no focused studies done yet that prove this one way or the other but the ones we do have indicate the difference in wear over a normal OCI are small. If you were to buy expensive oil filters over the life of a vehicle, the payoff for the double or triple expenditures on filters would likely not come.
Some of us like to hedge our bets under the "no engine ever failed because the oil was too clean" rule. We look for sales and good deals on the high efficiency filters. It's our fetish but some of us can find ROI justification for it. The best reason for a high efficiency filter, particularly with a full synthetic media, is the increased ability to run extended OCIs (>10k OCIs). Cleaner oil last longer. More pointedly, synthetic filters have a higher capacity per square inch of media. That is the way to make the premium filter pay off, though we have see again and again that even the "average" filters are capable of long runs (10K+) when the operating conditions are good. I think the safety factor is reduced and one shouldn't go that route unless it is well marked and researched.
