Quickie (ooh ahh missus!) - crush washer size and material?

[Dave48]

Out of interest, why do people think it's so important to replace it?
I've done dozens of oil changes, never changed the washer, never had even a drip.
It's not like there's any pressure behind it.

Its just good workshop practice. Youre tightening a steel bolt into alloy -hopefully to the right torque setting-&as the name suggests, its a crush washer designed to "spread" behind the bolt head forming a good seal. For the sake of a few pennies its peace of mind.
Probably more damage is done by overtightening components-its very common to see threads in alloy damaged by steel bolts/screws.