This filter was buried under soil.
As you can see it has corroded from the outside in. Had this not been discovered, it could possibly have led to a major oil spillage.

This highlights the importance of having the filter cleaned during servicing.

Never enclose the fuel filter, and make sure it’s never buried. Always ask your service engineer to check it.

​When having your boiler serviced, ensure that your service engineer cleans the fuel filter. Contamination in the fuel supply is one of the major causes of burner breakdowns. 
These pictures are from an appliance that was serviced regularly by another company.