[bigjae1976]

This is a tough one. I would not wait until the motor goes kaboom but its not cheap to address preventively. Can we monitor main bearing wear via oil analysis?

For track people who plan to keep an E46 M3 on track for awhile, it makes sense to keep a fresh spare motor on hand. I plan on annual motor swaps. Its not too bad as far as cost if you keep the internals stock. I plan on doing a basic check - leak down, rod bearing inspection, and look at the combustion chamber/bores with a boroscope.

Kind of harder to justify for an S65 since used motors are still $8kish and I'd be checking the mains too...which means total teardown.