As a general rule, the only real reason to ever change a filter is when it is loaded with contaminant to the point it is in danger of bypassing. As a filter is used and the contaminant gradually fills the pores in the element, the filter gets MORE efficient, never Less efficient. A properly designed filter will not push contaminant that is larger than the pore size thru the pores even though the pressure drop reaches the bypass valve setting. Since the Rotax recommendation of 25 hours when using 100LL is strictly because of lead contamination, mostly in the gearbox, it has nothing to do with the filter because the filter pore size freely lets the fine suspended lead pass right thru. Therefore, I personally believe there is no harm in filter changes at 50 hours.
