BTW, as to the function... If I recall correctly (and I may not because I don't have as many brain cells as I once did), when I changed mine I concluded that the o-rings separate the oil flow to the two parts of the filter. The diesel filters have a full-flow section and a bypass section. All the oil going to the bearings passes through the full-flow section unless the filter is plugged or the oil is too cold to pass through it, in which case a bypass valve - no relation to the bypass filter - will allow unfiltered oil to the bearings, as unfiltered oil is better than no oil. The bypass filter is a much finer pore size and gets a smaller amount of oil flow that I think is simply returned to the sump. The idea of the bypass filter is that it keeps cleaning up the oil in the sump, mainly for soot control. The full-flow section protects the bearings from the big chunks. If the o-rings are worn or missing, the oil flows will mix and the filter will not do its job properly. I don't think it is catastrophic, but it isn't good.

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1987 W201 190D