Both penetrate rather than film, and they split on colour chemistry:

Decking oils are penetrating blends (natural and modified oils) that feed the timber, repel water, and leave the wood looking like richer wood — clear to lightly tinted. They enhance without hiding, wear invisibly (no peeling possible), and re-coat with nothing but a clean and a dry day. Their limit is UV: clear oil barely slows greying, because it is pigment that blocks light.

Decking stains carry meaningful pigment in a penetrating (or lightly film-building) base: colour choice from subtle to solid, and genuine UV protection scaling with opacity — a mid-tone stain keeps boards brown for years where clear oil concedes the first summer. The trade: more pigment means less visible grain, and heavily film-building deck 'stains' inherit film problems (wear paths, flaking at board edges).

The working answer: hardwood decks bought for their looks (teak-toned ipe, balau) get oil and accept managed silvering or annual recolouring; softwood decks needing colour and UV defence get penetrating stain; and the popular middle path is an oil-based tinted decking product — oil behaviour, stain-grade UV. What consistently disappoints outdoors flat underfoot is glossy film-formers: decks flex, pool water, and grind grit — films fail here that survive on fences.

Preparation decides whether the coat lasts two seasons or one wet winter:

1. Clear and clean. Furniture and planters off; sweep; then wash with a deck cleaner that lifts grime, algae, and the grey oxidised surface fibres — scrubbed with a stiff broom or a moderate pressure wash (wide fan, moving, with the grain; aggressive blasting furs softwood).
2. De-grey where colour matters. A wood brightener/restorer (typically oxalic-based) after cleaning re-warms silvered timber and neutralises cleaner residues — the difference between re-oiling grey boards and restoring brown ones.
3. Strip failures. Old flaking film-stain must come off (stripper or sanding) before penetrating products can penetrate; sound old oil just needs cleaning.
4. New decks wait. Fresh timber — especially treated softwood — needs weeks of weathering so mill glaze and treatment moisture clear; the water-bead test (droplets soaking in rather than beading) says it's ready.
5. Dry properly: typically 24–48 rain-free hours after washing, moisture below ~18% if you meter it; oil over damp boards traps the damp.
6. Light sand of rough or furred areas (80–120 grit), sweep, and check the forecast — application plus 24 hours dry is the booking.

Skipped cleaning is the classic failure: oil over algae feeds the algae a sealed greenhouse, and next spring's 'the oil went black' was decided today.

Plan on annually for oils, every 2–3 years for pigmented stains — then let the deck itself set the true schedule, because exposure moves the numbers more than the tin does.

The variables: full-sun south faces weather at double the rate of shaded returns; high-traffic walk lines wear first; horizontal boards take UV and standing water that vertical cladding never sees; and softwoods drink and shed faster than dense hardwoods. A sheltered hardwood deck may cruise 18 months on oil; an exposed softwood stair can want attention every spring.

Read the deck, not the calendar: re-coat when water stops beading and starts soaking matte into the surface (the repellency test — thirty seconds with a cup of water), when colour visibly lightens or silvers, or when boards look dry and open-grained. Catch that window and re-coating is trivial — clean, dry, one refresh coat. Miss it by a year and you're back to full prep: cleaner, brightener, possibly stain.

Two habits that stretch every cycle: wash the deck each spring even in non-coating years (biofilm off, repellency inspected), and keep planters on feet with clear drainage paths — the rot and coating failures live under the pots, where the schedule never looks.

The greying is sunlight, not disease. UV breaks down lignin — the brown polymer binding surface wood fibres — and rain rinses the degraded surface away, leaving a thin silver-grey skin of cellulose. Every uncoated timber does it, hardwoods and softwoods alike; it is cosmetic at first, though the same open, degraded surface then holds moisture and algae, so long-neglected grey decks do drift toward genuine wear.

Recovery is routine: the grey layer is fractions of a millimetre. A deck cleaner scrub followed by a wood brightener/restorer dissolves and lifts the dead fibres, revealing the original colour underneath — the before/after that restoration products photograph. Stubborn or long-grey decks may need a light sand. Then protect the revealed colour immediately: tinted oil or pigmented stain, because the UV that greyed it is still on schedule, and pigment is the only real sunscreen (clear oil returns richness but delays greying only modestly).

Or keep it. Silvered timber is a legitimate finish — coastal and modern schemes cultivate it deliberately. Even a deliberately-grey deck should still get periodic clear oil or water repellent, though: the argument was about colour, but the water protection was never optional. Choose managed grey or maintained brown — the failure mode is only the unmanaged middle.

Compatibility runs on one principle: penetrating products need access to wood fibres.

Oil over old penetrating stain — yes, once the stain has weathered enough to absorb (water soak-in test). The stain's remaining pigment shows through, so expect tinted results; clean thoroughly first.

Penetrating stain over old oil — usually yes for oil-based stains onto weathered oil (fresh oil repels: wait a season or degrease); the existing tone darkens the delivered colour, so test a board.

Anything penetrating over a FILM-forming coating — no. Film-stains and decking 'paints' seal the surface; oil applied over them sits and stays sticky. The film must be stripped or sanded off first — the single most common decking-product mistake.

Water-based over oil-based: only after full weathering/cure and with products stated compatible; same-carrier systems always simplify life.

The diagnostic that answers every case in thirty seconds: pour water on the cleaned deck. Soaks in dull — penetrating products will take. Beads and sits — whatever is there is still sealing, and new coats will lie on top as a future flake. When switching systems, the same-brand pathway (their stripper, their prep, their product) converts most gracefully; and whatever the history, one test board before twenty square metres remains the cheapest insurance in the category.

Flat, absorbent, gappy timber has its own technique:

1. Tool: a decking applicator pad on a pole is the workhorse — fast, even, back-friendly, and it works product into the surface; brushes for edges, board ends, and between-board reaching; rollers only paired with back-brushing (they lay product on top; the brush pushes it in). Sprayers speed big decks but need immediate back-brushing and a windless day.
2. Method: work 2–3 full boards at a time end to end — never stop mid-board; the lap mark is permanent. Thin, worked-in coats along the grain; oil the cut ends (they drink and split first); catch drips between boards before they dry as runs on the joists.
3. Quantity discipline: penetrating products obey the wood's appetite, not your generosity — apply what absorbs in the stated window and wipe or brush out any surplus sitting proud after 15–30 minutes; puddled oil dries sticky and sheds dirt. Two thin coats (wet-on-wet if the tin allows) beat one flood.
4. Conditions: 10–25 °C, dry boards, dry forecast for 24 h, and out of direct midday sun, which flashes carriers off before penetration — morning shade is the professional's slot.
5. Aftercare: light foot traffic per tin (often overnight), furniture 24–48 h, and oily rags laid flat to dry or soaked in water before binning — bunched oil rags genuinely self-ignite, the one safety line every oiling day should respect.

Structurally, modern decks mostly survive without it; cosmetically and long-term, oiling earns its keep — and 'new' decks have special timing rules.

Treated softwood carries preservative against rot and insects — but preservative does nothing for UV grey, surface checking, or water cycling that opens splits along the grain. Oil or stain is what keeps treated boards looking chosen rather than weathered, and slows the split-and-splinter surface ageing that ends decks' comfortable barefoot years. Timing: let new treatment weather and dry (weeks to a few months) until water soaks in before the first coat.

Hardwoods (ipe, balau, teak-family): dense enough to endure unfinished for decades — which is why 'never oils it' is a legitimate school: the deck silvers evenly and lives on. The oiling school keeps the chestnut-brown colour with dedicated hardwood decking oils (thin, high-penetration formulas — standard oils sit on dense timber and stay tacky) at the cost of joining the annual-refresh club, since hardwood colour retention is the highest-maintenance look in decking. Either school still benefits from periodic cleaning and a water-repellent regime — the fastener stains and end-checking respect no philosophy.

Composite decking: no oil, ever — just cleaning; oils sit on capped boards as a film problem.

The honest framing for a new-deck owner: the first-year decision is not 'protect or not' but 'which look will you maintain' — brown on a schedule, or silver with a wash. Both are fine decks; only the drifting default (brown intentions, no schedule) disappoints.