What's left is a used oil soaked filter with all sorts of gunk collected from the oil.

Ostensibly you can monitor pressure in the oil filter canister over time and replace the filter as the pressure increases. You'll have to compare over time since canister pressure varies with rpm and temperature. Alternatively you can monitor flow and/or pressure in the bypass line. An increase suggests limited flow through the filter.

Sixto
87 300D^2