On a high efficiency filter, the calculations basically show that if there is 15% volume leakage, then the impact on efficiency is a 15% drop. If it was a 5% leak, then the efficiency drops by 5%. This was calculated simply by looking what the effect of the leakage would be based on upstream vs down stream particle counts. The leakage was estimated to be 15% on the gaps measured by
@Glenda W.'s mentioned in post 204. So a 99% @ 20u filter with a 15% volume leak past the media would drop the efficiency to 84% @ 20u.
If a filter with a metal-on-metal leaf spring to end cap interface was flat and smooth then the leakage would be minimal and the efficiency impact would be very small. But based on some of the photos of that seal area showing big gaps there's definitely some leakage going on, and it will vary with the level of leaf spring ruffles and flatness. Some people may not care, thinking that the leaking dirty oil will get cleaned up with more trips through the filter, which is true, but the leak is constant and like having the filter in partial bypass all the time, and makes the filter less efficiency than it should be.