The oil has gone black (or thick) — is that normal wear or a warning?

Black and thick are two different faults with two different causes, and they are worth telling apart before you order anything. Black means the oil has been overheated or oxidised. Thick means something has been carried into it — polymer, powder, or a process vapour that has cross-linked.

Darkening alone is not an emergency. Mineral oil darkens gradually over a normal service life and that is expected. What matters is the rate: oil that goes from new to dark in a few hundred hours is being cooked, and the usual causes are a restricted exhaust, a blocked oil mist filter raising back-pressure, a failed cooling fan, or simply an ambient temperature the pump was never rated for. Fix the heat, not the oil.

Thickening is the more serious one. Oil that pours like syrup no longer reaches the far end of the bearings and the vane slots, and the pump will seize before the ultimate pressure looks obviously wrong. Thickening usually means the process is sending something condensable into the pump — monomers, solvents that polymerise, or fine solids. An inlet trap sized for that process is the fix; a shorter oil-change interval only buys time.

Check the exhaust filter at the same time, every time. A loaded mist filter raises the pressure the pump exhausts against, and a pump working against raised back-pressure runs hotter, consumes more power and degrades its own oil faster. Many filters carry a back-pressure indicator precisely because this failure is invisible otherwise.

Do not judge the oil by colour alone once the pump is on a dirty duty. On those, the meaningful test is the pump's own ultimate pressure with a blanked inlet, measured on a gauge you trust, compared with the figure in the manual. That number tells you whether the oil is still doing its sealing job; colour does not.

Tell us the pump model, the process, the ambient temperature and how many hours are on the current charge. We will recommend the grade — mineral, ester, PFPE or silicone — and say when an inlet trap will save you more than cheaper oil will.