Filter test
A good way to see how dirty your filter is is to hold it up to a light and if you can see the light through the filter (just a glow) it is probably ok. If you can't see a glow from the light it is past due for a change. This test will work for a paper type element.
If you want to keep your Z stock, go with K&N filters and wash and re-oil every 10k miles.
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