If you have the same tranmission as a 1989 420 SEL, then it is probably the O-ring on the sender guage. In fact, I think there maybe 2 rings. I dropped the lower oil pan to replace them. You'll also need a new oil pan gasket. Dropping the pan is a little tricky. There are 2 nuts on the pan screws around the AC compressor. Also, be sure to get the torque on the pan screws correct. There are other posts here, that describe the job.

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1989 420 SEL