You don't trust a test lab that is ISO certified? Every one of those filters showed a decreases in efficiency as the delta-p went up. Why wouldn't all oil filters?On those particular filters. Was the data peer reviewed? How many other oil filters are made?
If an oil filter has a pretty low ISO efficiency, the fact that is can lose efficiency with loading is most likely why the ISO efficiency is lower. The ISO 4548-12 test measures the average efficiency from new to max holding capacity. So if the efficiency decrease as is loads up, then its overall ISO efficiency will be lower.
As mentioned, filters with higher ISO efficiency will hold their efficiency better as they load up than filters that have lower ISO efficiency. What was seen in Andrew's testing on all those oil filters is what is shown below.
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