I try to change out the O2 sensor with every other spark plug change. They get contaminated with fuel deposits/additives and any kind of silicone spray/fumes kills the sensor.
As far as the timing mark, it's possible the outer ring is slipping because of a loose/rotten rubber ring in the harmonic balancer. They dry rot and break loose causing the timing mark to drift from its correct location
91 YJ Sahara, 4.0L, AX-15/NP231, D30/D35@3.07
12 Cherokee 4x4,3.7L,42RLE/MP1522, D30Mfirstname.lastname@example.org
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