“How many hours before an oil change” is the most common question and the easiest one to answer badly. The same model can run a year in a clean, dry lab and emulsify in three months on a steam line. Change by the hour meter and you either throw oil away early or run it until the chamber scores.
Volume first. V0025A holds 0.75 L, V0040A 1.5 L, V0063 and V0100 2 L, and V0160 through V300B hold 5 to 8.5 L. A small sump degrades faster for the same amount of condensate, so a 0.75 L pump usually wants shorter intervals than a large one.
Four duty classes. Clean and dry duties (electronics, packaging, pick-and-place) age oil mainly by oxidation — watch colour and viscosity. Steam-bearing duties (cooking, drying, evacuation after washing) emulsify the oil — watch for cloudiness and a rising level. Solvent duties (printing, coating, degreasing) thin the oil and pull ultimate pressure down. Dusty duties load the sump with particles and accelerate vane wear.
Four signals you can act on. The sight glass should read pale yellow to amber; black, milky or layered oil is due. A drop of oil on filter paper, left 30 minutes, shows a dark centre with a wide spread ring when carbon or water is present. Compare ultimate pressure on the same gauge: a full step worse than with fresh oil (0.1 mbar drifting to 0.3 mbar) means change the oil before blaming the pump. A rising level usually means condensate; a sour smell means oxidised oil.
Change early after corrosive gas or heavy water vapour, a stall or an overheat event, a failed inlet filter that let dust in, and before restarting a pump that has stood more than three months.
Doing it properly. Run 5–10 minutes to warm the oil so contaminants stay suspended and drain out; stop, isolate and let the body cool; flush a little fresh oil through the inlet while turning the shaft by hand, then drain that too; fill to the centre of the sight glass, not to the top, since overfilling raises oil consumption and exhaust restriction; run 10 minutes, check for leaks, and note the level as your baseline.
One precondition for wet duties. Frequent changes will not save a pump handling water vapour, because the vapour condenses inside. Run with the gas ballast open. On V0040A through V0100 that moves ultimate pressure from 0.1 mbar to 0.5 mbar — the price of the ballast, and a trade to make deliberately against what the process really needs.
Treat the oil change as a condition-based decision, not a calendar entry: two of the three signals — oil sample, ultimate pressure, oil level — and it is time.