Pressure washers actually need two different types of oil depending on the model: engine oil (for gas units, same as any small engine) and pump oil (for the pressure pump itself, which has no filter and requires non-detergent formulas to avoid circulating contaminants).
Simpson Premium Pump Crankcase Oil 80138
Simpson’s own proprietary 15W40 pump oil blend, formulated to be non-foaming and specifically protective of the components inside Simpson pumps — the manual notes a generic SAE 15W40 is an acceptable substitute if the branded version is unavailable.
Honda 10W30 Small Engine Oil
For the engine side (not the pump) of any Honda-powered gas pressure washer, Honda’s own small-engine oil is the manufacturer-recommended standard, formulated specifically for the GX series engines used across Simpson, Generac, and other brands’ gas models.
Valley Industries Premium Pump Oil
A widely compatible non-detergent pump oil suited to most residential and light-commercial pump brands beyond just Simpson, offering the same non-foaming, rust- and oxidation-resistant protection manufacturers specify for pump longevity.
Check your owner’s manual to confirm whether you are maintaining the engine or the pump — they take different oils. Use Honda 10W30 (or your engine brand’s equivalent) for the engine, and a non-detergent pump oil like Simpson’s 80138 or Valley Industries’ Premium Pump Oil for the pump itself.
Checking and changing each one correctly
The engine oil on a gas pressure washer is checked and changed exactly like any small engine — dipstick or sight glass, drain plug on the underside, and a change interval usually printed in hours of use (commonly every 25-50 hours, or at least once a season for light homeowner use) rather than a fixed calendar date. The pump oil is a separate reservoir entirely, usually with its own small sight glass or fill plug near the pump housing, and most manufacturers recommend changing it after the first 50 hours of use (to flush out initial break-in wear particles) and then annually or every 100 hours after that for continued light use. A simple way to remember which is which if your manual’s language is confusing: if the oil touches anything that burns fuel and spins a crankshaft, it’s engine oil and needs a detergent formula; if the oil touches the pump that actually pressurizes water, it’s pump oil and needs a non-detergent formula. Mixing the two up — using pump oil in the engine or engine oil in the pump — causes real problems either way, so double-check which reservoir you’re filling if your particular machine’s layout isn’t obvious at a glance.
Frequently asked questions
What happens if I accidentally put engine oil (detergent) in the pump?
Since the pump has no filter, the detergent additives stay in circulation rather than being trapped, which over time increases wear on seals and internal surfaces. A single accidental use likely won’t cause immediate failure, but it should be drained and replaced with correct non-detergent oil as soon as you notice the mistake.
Do electric pressure washers need any of this oil maintenance?
No — electric pressure washers don’t have a gas engine, so there’s no engine oil to maintain, and most electric-unit pumps are sealed and lubricated for the life of the pump rather than user-serviceable. This oil maintenance discussion applies specifically to gas-powered machines.
How can I tell if my pump oil needs changing just by looking at it?
Check the sight glass if your pump has one — oil that looks milky or cloudy has likely absorbed water (a sign of a seal issue worth investigating), while oil that’s turned very dark has accumulated wear particles and is due for a change regardless of the hours-based schedule.
Does climate affect which viscosity I should use for either the engine or the pump?
Yes — colder climates generally benefit from a lower-viscosity winter-weight oil (or a multi-grade formula like 15W40) that flows more easily at startup in cold temperatures, while consistently hot climates can tolerate a higher single-grade viscosity without issue. Check your manual for climate-specific recommendations rather than using the same oil year-round if you experience significant seasonal temperature swings.
Winterizing matters as much as routine oil changes
Beyond the engine/pump oil distinction, seasonal storage is where a lot of avoidable pump damage happens — water left inside the pump over a freezing winter expands as it turns to ice and can crack internal components, which is a separate issue from oil type entirely but often gets lumped into the same “pump maintenance” conversation. Running pump antifreeze/storage fluid through the system before winter storage, or at minimum draining all water from the pump and hose, protects against this regardless of which oil you’re using. If you live somewhere with real winters and store a gas pressure washer in an unheated garage or shed, treat winterizing as a non-negotiable step alongside whatever oil-change schedule you’re already following. A quick end-of-season checklist worth keeping handy: run the engine out of fuel (or add stabilizer), drain all water from the pump and hose, apply pump antifreeze if the manual recommends it, top off both oils to their correct levels, and store the unit somewhere it won’t sit in standing water or direct exposure to the elements until spring. Skipping this routine even once in a genuinely cold climate is a common way an otherwise well-maintained machine ends up with a cracked pump housing come spring, which is a far more expensive fix than the twenty minutes winterizing actually takes.
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