Volvo S40 Oil Pressure Warning

Indeed, exceeding the maintenance interval on a diesel engine can cause significant oil sludge buildup. This leads to oil contamination and reduced oil pressure, consistent with the symptoms you describe. Has the oil filter been changed recently? What does the oil look like on the dipstick?

The oil filter was changed during the last service, about 15000 km ago. The oil on the dipstick looks dark and seems thicker than usual. Could this indicate more serious internal damage?

The dark and thickened oil strongly suggests advanced contamination. Although it's difficult to ascertain without inspection, the extended interval likely caused excessive wear. Continuing to drive could lead to engine failure. I recommend a qualified technician assess the situation immediately.

Thanks for the advice. It was indeed the exceeded maintenance interval. An oil change and new filter, costing 165€, resolved the issue.