Used Motor Oil Instead of Stain – Is This a Good Idea?

The claim, which showed up in my Facebook feed recently, is this: used motor oil is a great wood treatment for exterior siding. It adds color, it darkens beautifully over time, it protects the wood, and — best of all — it’s free. The person posting this had gotten the tip from their 92-year-old ex-landlord, who I think of a Betty.

The old ways, though, are not always the best ways. Let me explain why this particular piece of folk wisdom belongs in the same category as bloodletting and leaded gasoline.

The Fire Hazard

Motor oil soaks into wood and doesn’t cure. Unlike purpose-made penetrating finishes, which polymerize and harden inside the wood fibers, oil just sits there — wet, combustible, indefinitely. That lowers the wood’s ignition point and means that when it does catch fire, it burns hotter and faster than untreated wood. For most of the people sharing this tip online, that risk probably feels abstract. But if you are living in a tiny home on wheels and the structure you are treating is also your home, you may want to think about this differently than someone refinishing an old outbuilding.

There’s also the application hazard. Rags soaked in drying oil — motor oil included — are a classic cause of shop fires. As the oil oxidizes, it generates heat. A rag balled up in a trash can can get hot enough to ignite on its own, with no spark required. This is the reason every can of oil-based finish includes a warning about disposing of rags by spreading them flat outdoors until dry.

The Legal and Environmental Problem

Used motor oil is classified as hazardous waste in most U.S. jurisdictions. It accumulates heavy metals, combustion byproducts, and other contaminants over its service life. Using it as a wood treatment — where it will leach into soil every time it rains — is either explicitly prohibited or strongly discouraged in most states, Washington included.

It Doesn’t Even Work That Well

Here is the part that really undermines the folk wisdom: motor oil is not actually a good wood preservative. It never fully cures, which means it stays tacky and attracts dust and grime. It provides minimal UV protection, so the wood underneath continues to degrade from sun exposure even while it looks dark and treated. Purpose-built penetrating finishes — which have existed for decades and are not expensive — outperform it in every measurable category.

Now, there’s a narrower version of this claim that deserves a little more credit. Some old-timers, particularly barn builders, used clean hydraulic oil or non-detergent motor oil — not used, dirty oil — on rough-sawn exterior wood. That’s a different proposition. New hydraulic oil is lighter, goes on smooth, and doesn’t carry the contamination load of used oil. Some people report decent results lasting four or five years. I wouldn’t recommend it on a home you are living in, but I understand the tradition, and it’s at least not the same thing as draining your crankcase onto your siding.

Used motor oil is the worst version of this idea.

What to Actually Do with Natural Wood Siding in the PNW

If you have cedar, fir, or another natural wood exterior — common on tiny homes, outbuildings, and plenty of older houses in the Pacific Northwest — here is what actually works in a wet climate.

The fundamental rule is: penetrating oil finish, not a surface coating. Film-forming products like paint, solid stains, and clear sealers look great on day one and then fail catastrophically when moisture gets underneath, which it will, because this is the PNW and moisture gets into everything. When a film coating fails, it peels, blisters, and traps water against the wood. A penetrating oil finish has nowhere to peel from. It lives inside the wood fibers, not on top of them. When it wears out, it just fades and the wood looks dry, which is your cue to refresh.

Good products for this climate: Penofin (marine grade is the recommendation from builders who work in cold, wet climates), Ready Seal, which absorbs evenly without flaking or blistering even on wood with raised grain from weather exposure, and Cabot’s Australian Timber Oil, which has a long track record on cedar in high-moisture environments. For Western Red Cedar specifically, semi-transparent oil-based penetrating stains are the best choice. They offer protection while still showing the wood’s natural character, and applied correctly to textured surfaces, can last three to six years before reapplication.

A few things worth knowing before you start:

Don’t apply in full sun. Working in the shade gives the stain more time to penetrate before the surface dries. On a July afternoon in Bellingham you might have a two-hour window before the surface is too warm.

Two coats beats one. Two coats of penetrating oil-based stain on textured cedar will provide longer service life than one coat, but only if the wood will actually accept the second coat. Apply the second coat while the first is still slightly tacky, usually within an hour or two.

Refresh on a two-to-three year cycle, not five. Product manufacturers tend to advertise five-year intervals because it sounds impressive on the label. In Western Washington, where you are dealing with nine months of moisture and limited drying time, that’s optimistic. Three years is more realistic, and it’s much easier to refresh a surface that still has some protection left than to rescue one that has gone gray and checked.

Every spring: light power wash and inspection. You are looking for any spot where the wood has gone dark and stayed dark. That’s moisture that isn’t drying, usually because something above it (trim, a window flashing, a roof edge) is channeling water where it shouldn’t go. Catch those spots in April and you have all summer to fix them. Find them in November and you are patching in the rain.

Betty’s motor oil would have failed all of these tests. But I’m sure she was right about the vegetables.

Mount Baker Tiny Homes builds custom tiny homes on wheels and van conversions in Bellingham, Washington. Get in touch if you want to talk through a build.