Without a bolt, the engine will have reduced power which may result in a stall, increasing the risk of a crash.

The rocker arm shaft retaining bolts may have been manufactured incorrectly resulting in the bolts loosening during engine operation. A loose bolt will initially result in a slow leak of engine oil, but if not addressed, the bolt can come out of the cylinder head entirely.

What to do: Book it in at your next service. There is no deadline and no charge.