The probability of the oil filter cover nuts getting overtorqued is really high. Even if you didn't do it, previous oil changes by previous owners at "time-saving" lube shops increases the possibility even more. The nut was probably on its last thread and gave up.

The recommended torque rating is only 20 Nm/ 15 Ft-lb. which is not much! Basically, just enough to slightly compress the o-ring. I've found that Fastenal has the sizes of double-end stud and specialized nut needed.

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Sam

84 300SD 350K+ miles ( Blue Belle )