The engine oil system: what every part does

Every moving part in your engine runs on a thin film of oil, pumped around a loop the whole time the engine runs. Most oil-related failures start with a small part nobody looks at: a strainer, a rubber flap, a washer, a breather valve. Here's the loop, stop by stop: what each part does, how it fails and what you can check yourself.

Sump and dipstick

What it is. In a wet-sump engine, which is what most road cars have, the sump (oil pan) holds the whole oil supply and has to keep the pickup submerged. Baffles trap oil around the pickup in hard corners and under acceleration.

What fails. Pans leak at the gasket and drain thread, and get dented from below.

What you check. Read the level the way your handbook says, because carmakers differ:

• Nissan: level ground, warm up (about 5 minutes at idle if cold), switch off, wait at least 15 minutes.
• Hyundai: also waits 15 minutes for the oil to drain back. Toyota UK checks a cold engine on level ground.
• Never fill above max.

Nissan says some topping up between services is normal, but for its engines more than 0.5 L per 1,000 km after the first 5,000 km needs a workshop check. Cars with an electronic level sensor usually show a warning lamp, a message and a fault code when it fails.

Pickup and strainer

What it is. A short pipe with a wire-mesh strainer at the bottom of the sump. The pump drinks through it. The mesh is coarse because cold oil is thick, and even a thin layer of deposits can cut flow sharply.

What fails. Clogging, from sludge or from gasket sealant squeezed out in an earlier repair. Audi's R8 bulletin describes excess sealant blocking the oil pump strainers; the fix replaced all eight.

What you check. With the pan off, look at the screen, and use sealant sparingly on reassembly.

Oil pump and relief valve

What it is. The engine drives the pump, which pushes oil from the pickup through the filter and into the engine. Common designs are gear, gerotor and vane pumps. MS Motorservice says the full oil volume has to go round the engine 4 to 6 times per minute. A spring-loaded relief valve opens above a set pressure and sends the excess back to the pump inlet or the sump.

What fails. Wear, debris and starvation. Lost oil pressure means bearing damage, seized pistons and a dead turbo.

What you check. You can't see the pump without a teardown. Watch the warning light, keep the level right and change oil and filter on time.

Oil filter

Bypass valve

What it is. A spring valve that lets oil skip the element when the filter is clogged or the oil is very thick from cold, so the engine isn't starved. Unfiltered beats none, but Premium Guard warns that a valve opening too soon or too often sends dirty oil in and can speed up wear.

What fails. The wrong filter. Opening pressure is set per filter: FRAM lists 12 psi (about 0.8 bar) for the PH16 and K&N 11 to 17 psi for its HP-1004. Some engines put the bypass in the block or filter housing instead, so the filter has none.

What you check. Fit the catalogue part number for your engine, not a lookalike with the same thread.

Anti-drainback valve

What it is. A rubber flap that keeps oil in the filter when the engine is off, so oil reaches the bearings quickly at start-up. Filters mounted sideways or upside down also need an anti-syphon valve.

What fails. A FRAM-sponsored trade video says heat and dirty oil can harden the flap over time. GM's bulletin 10-06-01-003C goes further: on its V6 engines, deteriorated anti-drainback valves in aftermarket filters caused oil starvation, noise and internal damage. GM's advice: use the correct part number and "not rely on physical dimensions alone".

That cold-start rattle. The internet blames the flap. Carmakers' own cold-start rattle bulletins mostly blame the cam-timing actuators, timing chain or pistons. Fit the right filter first; if the rattle stays, get the phasers checked.

Fitting it

• Spin-on cans hold the anti-drainback valve; some cartridge systems put it in the housing.
• Get the old gasket off. MAHLE warns two stacked gaskets can leak or stop the valve opening fully.
• On cartridge housings, Pierburg says to replace the cap's sealing rings every time. On Mercedes OM611, OM646 and M271 engines, a cracked cap tube can dump pressurised oil back to the sump and wreck the bearings and turbo.
• Tighten to spec. Hyundai gives 13.7 N·m (10 lb-ft) for its petrol engines; check your manual.

The full job is in how to change your oil.

Galleries, bearings, cam timing, turbo and oil cooler

Drilled passages carry filtered oil to the crank and rod bearings, the camshafts, the turbo and the variable valve timing. Starve any of them and you get bearing damage, piston seizure or turbo failure.

• Turbo feed: Melett lists coked feed pipes, silicone sealant on the oil inlet gasket, a low sump level, hot shutdowns and long idling as causes of turbo oil starvation. The bearings can fail within seconds.
• Cam phasers: oil pressure, metered by a solenoid, moves the phaser against a spring. Oil thicker than specified can set false VVT codes, and the solenoid screens can varnish up.
• Oil cooler: MAHLE describes it as a plate heat exchanger with two oil and two coolant connections. An internal leak shows as oil droplets in the expansion tank and light slime under the filler cap. The fix is a new cooler, an oil change and a coolant flush. A failed head gasket can look the same.

What you check. Oil film in the expansion tank, and no sealant on turbo oil-line gaskets.

Oil pressure switch and warning light

What it is. A normally closed switch: the red oil light comes on with the ignition and goes out once the engine builds pressure. Thresholds are low and engine-specific; one VW switch triggers at 1.4 bar.

It's not a level light. Nissan says it isn't designed to show low oil level, and to stop the engine immediately. VW: stop and check the level. If it's correct, don't drive on. If you topped up, restart for five seconds and carry on only if the light goes out.

What fails. A light that stays on or never comes on, and leaks at the switch.

What you check. On with the ignition, off after start.

Drain plug and crush washer

What it is. The bolt at the bottom of the sump and its sealing washer. Hyundai calls the washer a one-time-use crush gasket and says to check the old one isn't stuck to the pan. Subaru marks its gasket "ONE-TIME USE".

What fails. Threads and pans, mostly from too much torque or the wrong plug.

• Torque is vehicle-specific. In carmaker documents, engine oil drain plugs run from 8 to 50 N·m. On the engines BMW lists, 8 to 40 N·m, mostly 25 N·m. Hyundai's petrol engines take 39 N·m (29 lb-ft). Check your manual.
• Over-tightening can loosen the pan's threaded insert (Infiniti) or crack the pan around the drain boss; Stellantis says never to use power tools on the plug.
• Hyundai warns against generic plugs, because thread depth and flange size differ. VW traced leaks to the wrong plug and seal for the pan type.
• Audi's EA888 Gen 3 uses a plastic plug with a bayonet lock; follow the workshop information for it.

What you check. New washer, old washer off, right plug, your car's torque figure. The oil change checklist has it on one page.

PCV and crankcase ventilation

What it is. Combustion gas leaks past the piston rings (blow-by). The crankcase ventilation system vents it and controls crankcase pressure. It separates oil droplets and returns them to the sump. BMW's design uses a spring-and-diaphragm valve that keeps a slight vacuum in the crankcase.

What fails. A worn valve or split diaphragm lets pressure go the wrong way:

• Too much pressure: leaking seals, higher oil consumption that BMW says can be misdiagnosed as a bad turbo, oil in the intake and intercooler, a dipstick pushed out of its tube.
• Too much vacuum: rough idle or misfire and a whistling or howling noise.
• Torn diaphragm on a turbo engine: power loss, rough idle and whistling as boost escapes into the crankcase. On the Focus ST the vent is part of the oil filter housing and is replaced as a unit.

What you check. A whistle at idle, oily charge pipes, split breather hoses.

Residue under the oil cap

Toyota's bulletin T-SB-0104-21 explains the winter "mayonnaise": moisture from blow-by condenses in the oil during short trips in extreme cold. Mazda says the same and has dealers tell owners the engine needs to reach normal operating temperature to drive the moisture off.

Explainable is not normal. Both carmakers treat it as a fault and fix it with an oil and filter change.

Test the cooling system first. Both bulletins start with a cooling-system pressure test, because a coolant leak looks the same. Signs it's coolant, not condensation:

• bubbles in the expansion tank, overheating, coolant loss
• white exhaust smoke
• a block tester's fluid turning from blue to yellow (petrol) or green (diesel)
• oil in the expansion tank, which can also be the oil cooler

Residue only on the cap, after short cold trips, with steady coolant and a clean dipstick, points to condensation. Elring, a head gasket maker, notes many complaints blamed on the gasket have another cause.

Oil grades: what 0W-30 means

The W stands for winter and describes cold behaviour. The lower the W, the faster the oil flows at a cold start. The second number is viscosity at operating temperature.

Grades are set by SAE J300, latest revision J300_202405 (May 2024). The limits below are from the J300 table as published by Infineum.

Cold tests. The cranking test (CCS) tells you whether the engine can turn over and start. The pumping test (MRV) checks oil still reaches the pump after a cold soak, when wax can gel and starve it.

W grade	Cranking (CCS) max	Cranking test temperature	Pumping (MRV) max	Pumping test temperature
0W	6,200 mPa·s	−35 °C	60,000 mPa·s	−40 °C
5W	6,600 mPa·s	−30 °C	60,000 mPa·s	−35 °C
10W	7,000 mPa·s	−25 °C	60,000 mPa·s	−30 °C

Hot tests. At 100 °C, SAE 30 must be 9.3 to under 12.5 mm²/s and SAE 40 12.5 to under 16.3. There's also a minimum high-temperature high-shear (HTHS) viscosity at 150 °C, which stands in for the bearings and rings: too low risks wear, too high costs fuel. It's 2.6 mPa·s for SAE 20, 2.9 for SAE 30, and 3.5 for 0W-40, 5W-40 and 10W-40.

So a 0W-40 and a 10W-40 meet the same hot limits; the W only changes the cold tests. Same limits, not the same oil.

Wear. GM's radiotracer work found bore wear is dominated by break-in, cold starts and changing conditions. Use the W grade your manual lists for your climate and drive gently until warm.

Specs and approvals

The grade says how thick. The spec says what the oil is built for.

API and ILSAC (US). API SP and ILSAC GF-6 started in May 2020, adding protection against low-speed pre-ignition (LSPI), timing chain wear and for stop-start. API SQ and ILSAC GF-7 followed in March 2025.

ACEA (Europe). A/B categories are "High SAPS" (sulphated ash, phosphorus, sulphur) for cars without particulate filters. C categories are catalyst and DPF/GPF compatible: C4 is low-SAPS, the rest mid-SAPS. ACEA says they "will increase the DPF/GPF and TWC life". The ash a DPF can't burn off comes mostly from oil additives.

ACEA C	SAPS	Sulphated ash max	HTHS at 150 °C
C2	mid	0.8%	≥2.9 mPa·s
C3	mid	0.8%	≥3.5 mPa·s
C4	low	0.5%	≥3.5 mPa·s
C5, C6	mid	0.8%	2.6 to <2.9 mPa·s
C7	mid	0.8%*	2.3 to <2.6 mPa·s

*C7 shares a merged cell with C5 and C6 in ACEA's table.

C3 and C5 have the same ash limit; the difference is HTHS, 3.5 versus 2.6. A C5 oil in an engine that asks for C3 is a step down in hot viscosity, not a DPF choice. A7/B7 and C6 add LSPI protection for turbo direct-injection petrol engines.

Carmaker approvals sit on top:

• VW 504 00 / 507 00: long-life petrol and diesel; 507 00 is mid-SAPS, C3-type.
• VW 508 00 / 509 00: 0W-20 with HTHS of at least 2.6, not backwards compatible with older VW specs.
• Mercedes-Benz 229.51, 229.52, 229.71: low-ash sheets; 229.71 is mostly 0W-20.
• BMW Longlife: LL-01 for engines without a particulate filter, LL-04 with one, LL-17 FE+ for low-HTHS 0W-20 and 5W-20.
• GM dexos1 Gen 3: tougher deposit, turbo and LSPI requirements, backwards compatible with Gen 2.

One bottle can carry several: Mobil 1 ESP 5W-30 lists VW 504 00/507 00, MB 229.51/229.52 and BMW LL-04 and meets ACEA C3. Find the approval code in your handbook and match it on the bottle.

What to check yourself

• Level: handbook method, level ground, never above max.
• Consumption: note every top-up so you spot a change.
• Warning light: on with the ignition, off after start; if it lights while driving, stop.
• Oil cap: residue? Check coolant level and the expansion tank before blaming the head gasket.
• Breather: whistling, rough idle, oily charge pipes, a dipstick lifting.
• Oil change: catalogue-correct filter, old gasket off, new drain washer, your car's torque, no power tools.
• Oil: the grade and approval code in your manual.
• Interval: respect the time limit even on low mileage; short cold trips count as severe use.

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