What oil does a misting pump take, and how often should you change it?

A misting pump takes the oil named by the company that made its pump head, and on most 1,000 psi plunger pumps that oil is changed once at 50 hours and then every 500 hours. The brand on the misting kit is often not the brand of the pump head: the Aeromist direct-drive pumps, for example, list a Cat pump and call for Cat pump oil. Find the maker on the crankcase, then use the table below.

This applies to oil-bath plunger pumps, the type used on high-pressure misting systems. Diaphragm pumps on mid-pressure systems have no oil to change, for reasons covered in plunger vs diaphragm pumps.

What oil and interval does each pump maker specify?

The manuals agree on the shape of the schedule and disagree on the oil. Every maker that states a break-in calls for an early first change, because a new pump sheds wear particles while its parts bed in, then a fixed hour interval. What they put in the crankcase ranges from hydraulic oil to diesel-rated engine oil.

Crankcase oil and change intervals from plunger pump and misting pump manuals (read 2026-10-04)

Pump maker or unit	Oil specified	First change	Then every	Calendar limit
Cat Pumps (plunger pump heads)	Cat's custom-blend multi-viscosity ISO 68 hydraulic oil	50 hours	500 hours; 300 hours with any other oil	3 months, whichever comes first (5, 7, 15PFR manual)
Aeromist and Advanced Misting Systems direct-drive 1,000 psi pumps (Cat heads)	Approved Cat pump oil	Not stated	500 hours	Once a year
General Pump (pumps under 25 BHP)	Series 100, 30 weight, non-detergent	50 hours (oil data sheet: 50-100)	500 hours (oil data sheet: 300)	3 months
Udor plunger pumps, light duty	SAE 15W-40 engine oil, or equivalent	50 hours	500 hours; 250 hours where humid air condenses	Not stated
Hydromist Mist-n-Go portable (0.132 GPM, 850 psi)	ISO 68 or equivalent; gear oil ISO VG 68 or VG 100	50 hours	250-300 hours	Change before use after more than a year idle

Two patterns stand out. First, every maker that states a break-in change puts it at 50 hours. Second, the small portable unit runs roughly half the interval of the base-mounted pumps, so a schedule copied from a patio pump will run a portable pump's oil well past its limit.

Is it 300 hours or 500 hours?

For a Cat pump head, it is 500 hours with Cat's own ISO 68 oil and 300 hours with anything else. Cat's 3CP and 5, 7, 15PFR service manuals both carry the same footnote under the maintenance check-list: if other than Cat's special multi-viscosity ISO 68 oil is used, the change cycle should be every 300 hours. That footnote is the main reason owners and supplier blogs quote both numbers for the same pump. Neither is wrong; they describe two different oils.

General Pump adds a second source of confusion. Its oil recommendations sheet says 50 hours, then every 3 months or 500 hours. Its Series 100 oil data sheet says a break-in change between 50 and 100 hours, then every three months or 300 hours. When one maker's own documents disagree, use the shorter figure: oil is the cheapest consumable on the pump, and the sheet that says 500 hours also says failure to follow its requirements could void the warranty.

Which oil type should you buy?

Buy the oil your pump head's maker names. The manuals do not agree on one oil class, so there is no single "misting pump oil" that is correct for every pump:

• Cat Pumps recommends its ISO 68 high-pressure hydraulic pump oil, described in Tech Bulletin 134 as petroleum-based with additives against wear, oxidation, rust and corrosion. Its largest 150FR frame uses an ISO 100 oil instead.
• General Pump uses Series 100 oil, 30 weight non-detergent, on pumps needing less than 25 brake horsepower, and heavier Series 220 (50 weight) above that. Its troubleshooting table answers water in the crankcase with "use General Pump SAE 30 non-detergent oil".
• Udor specifies SAE 15W-40 or equivalent and lists 15W-40 engine oils by brand name, so motor oil is the correct oil in a Udor pump.
• Hydromist asks for good quality gear oil, ISO VG 68 or VG 100, and says to avoid PAG and ester oils, never to mix oil types, and to flush the crankcase with the new oil when switching type.

The general claim that motor oil ruins pump crankcases is therefore too broad. It is the wrong oil in a Cat or General Pump head because those makers specify something else, and in a Cat head it also cuts the interval to 300 hours. It is the specified oil in a Udor head.

How much oil does it take?

Less than most owners expect. Fill to the mark, not to a measured volume, since draining never empties the crankcase completely; the capacities below tell you how much to buy.

Crankcase oil capacity of pumps used for misting

Pump	Capacity	Changes per 21 oz bottle
Cat 1CX compact plunger head	2.03 oz (0.06 L)	10
Hydromist Mist-n-Go portable	2 oz (60 ml) maximum	10
Aeromist direct-drive, 0.33-1.5 GPM; Cat 1DX to 4DX heads	8.5 oz (0.25 L)	2
Aeromist direct-drive, 2.1-2.9 GPM; Cat 2SF series	10.1 oz (0.30 L)	2
Udor PN series	0.29 L (about 9.8 oz)	2
Udor PK series	0.36 L (about 12.2 oz)	1

The 21 oz bottle is the size listed for Cat pump oil in the Aeromist parts list and for Hydromist's ISO 68 oil. A typical patio pump therefore needs one bottle per season.

How many oil changes does a season need?

It depends on run hours, so count them. An hour meter on the pump's switched power is the reliable way; the controller run schedule gives an estimate.

How do you change the oil?

The procedure is the same on every oil-bath pump checked here; only the drain location differs.

1. Shut down and depressurize. Disconnect power and release pressure from the misting line before touching the pump. Hydromist's procedure starts with purging pressure and water and removing the supply and high-pressure hoses.
2. Mind the temperature. Interpump's Series 47 manual says to drain with the pump at operating temperature, which helps the oil flow out. Hydromist warns not to change hot oil because it can cause severe burns, and Udor says to wait for the machine to cool and to wear gloves. Warm is useful; hot is not.
3. Level the pump and drain it. Remove the dipstick or fill cap first so the oil drains freely, then the drain plug. Hydromist says to drain the old oil as completely as possible, because impurities left behind dissolve in the new oil.
4. Refit the plug. Udor suggests replacing the drain plug gasket every time.
5. Refill to the mark with the pump off. Aeromist and Advanced Misting Systems say half full on the sight glass, with the unit off, and not to overfill. Cat's bulletin says fill to the red dot on the bubble gauge; its PFR manual says slightly above center if the pump runs in reverse rotation. General Pump and Interpump say the middle of the sight glass or level indicator.
6. Check for leaks and log the hours. Run briefly, look at the drain and fill plugs, and note the hour meter reading.
7. Recycle the old oil. Hydromist notes many auto parts stores accept used oil for free.

What should the oil look like between changes?

Clear and at the mark. Cat's Tech Bulletin 134 says to check level and quality daily and gives the two colors that matter:

• Black can mean old, worn oil, overheated oil, or the wrong oil.
• Milky white means water has leaked from the pump manifold into the oil. Cat says to service the pump as soon as possible.

Water can also enter without a seal failure. Cat's troubleshooting table lists humid air condensing inside the crankcase, with an oil cap protector and the 3 month or 500 hour change as the remedy. Udor's answer to the same symptom is to change oil every 250 hours instead of 500. A pump in a damp enclosure or a humid climate (see misting in humid climates) is the case these notes describe. Oil that turns milky again soon after a change points to worn seals, and knocking or rough running alongside it is diagnosed in pump cycling and noise.

Should you change oil before winter storage?

We recommend it whenever a change would fall due early next season. None of the manuals says whether storage months count toward a 3 month limit, but two say oil ages without running: Hydromist says oil left unused for a long time deteriorates by oxidation and can lead to internal rusting, and General Pump's MKS manual says oil must be changed at least once a year for the same reason. Storing a pump on fresh oil removes the question. The rest of the storage routine is in winterizing a misting system.

When does this answer stop applying?

• Your pump's manual differs. The figures here come from the listed documents. Where your manual gives another oil or interval, it governs, and General Pump ties its requirements to the warranty.
• Heavy duty. Udor's 500 hour figure is for light duty and it says to service more often for heavy duty. Hydromist says to shorten intervals in excess heat, dust, rain, high humidity or constant overpressure.
• Large commercial pumps. Pumps above 25 brake horsepower in the General Pump range use a 50 weight oil, and its large MKS series uses 220-grade oils. The table above covers pumps sized for patios and restaurants, which the misting pump guide describes.
• Pumps without oil. Diaphragm pumps and sealed compact pumps have no oil change at all.

Oil is one item on the pump's service list; filters, nozzles and seals have their own intervals in the maintenance section.