Mopower Garage

What an Oil Filter Does

Published by Christopher J. Holley | Mopar History & Tech | January 2026

Engine oil picks up:

  • Metal wear particles
  • Carbon soot
  • Dirt and dust
  • Oxidation byproducts (sludge, varnish)

The oil filter captures these contaminants while allowing clean oil to circulate under pressure to bearings, camshafts, turbochargers, and valvetrain components.

If the filter fails or clogs:

  • Oil flow can be restricted
  • Bearings can starve
  • Engine wear accelerates rapidly

Main Types of Automotive Oil Filters

1. Spin-On Filters (Most Common)

  • Self-contained metal can
  • Screw directly onto engine block
  • Includes filter media, bypass valve, and anti-drainback valve
  • Easy and quick to replace

Pros: Simple, inexpensive
Cons: Entire unit discarded each change

2. Cartridge Filters

  • Replaceable filter element only
  • Housed in a permanent plastic or metal housing
  • Common in European and newer vehicles

Pros: Less waste, easier inspection
Cons: More sensitive to incorrect installation

3. Magnetic Filters

  • Use magnets to attract ferrous metal particles
  • Often supplemental rather than primary filtration

Pros: Captures metal wear debris
Cons: Does not trap non-metal contaminants

4. Centrifugal Filters (Rare in Passenger Cars)

  • Use centrifugal force to separate contaminants
  • Common in heavy-duty diesel engines

Internal Anatomy of an Oil Filter

Key Components

  1. Filter Media
    • Paper (cellulose)
    • Synthetic
    • Blended
  2. Bypass Valve
    • Opens if filter is clogged or oil is too thick (cold starts)
  3. Anti-Drainback Valve
    • Prevents oil from draining back when engine is off
    • Reduces dry starts
  4. End Caps
    • Metal or fiber; metal is more durable
  5. Center Tube
    • Structural support for oil flow
  6. Gasket
    • Seals filter to engine block

Filter Media Types

Cellulose (Paper)

  • Traditional
  • Filters larger particles (≈20–40 microns)
  • Lower cost
  • Shorter service life

Synthetic

  • Uniform fiber structure
  • Filters smaller particles (≈10–15 microns)
  • Higher dirt-holding capacity
  • Ideal for extended oil change intervals

Blended Media

  • Balance of cost and performance
  • Most mid-range filters use this

Micron Rating (Filtration Precision)

  • 1 micron = 0.001 mm
  • Human hair ≈ 70 microns

Typical automotive oil filters:

  • Standard: 20–40 microns
  • Premium synthetic: 10–15 microns
  • Racing/high-performance: sometimes looser to ensure flow

Smaller micron rating = better filtration, but flow must still meet engine requirements.

Bypass Valve: Why “Unfiltered Oil” Is Better Than No Oil

When oil is:

  • Cold and thick
  • Filter is clogged
  • High RPM demands exceed flow capacity

The bypass valve opens to prevent oil starvation.
This is normal and intentional—not a defect.

Anti-Drainback Valve (ADBV)

  • Usually silicone (better) or nitrile rubber
  • Critical for engines with:
    • Overhead cams
    • Oil filters mounted horizontally or upside down

A failed ADBV can cause:

  • Startup rattles
  • Delayed oil pressure

Oil Filter Lifespan

Depends on:

  • Filter quality
  • Oil type
  • Driving conditions
  • Engine condition

Typical intervals:

  • Conventional oil: 3,000–5,000 miles
  • Synthetic oil: 7,500–15,000 miles (with high-quality filter)

Never reuse an oil filter.

OEM vs Aftermarket Filters

OEM filters

  • Designed for specific engine oil flow and pressure
  • Often very good quality

Aftermarket filters

  • Range from budget to high-performance
  • Some exceed OEM specs; others cut corners

Key differences may include:

  • Media quality
  • Valve materials
  • Can thickness
  • Burst pressure rating

High-Performance & Racing Filters

Designed for:

  • Higher flow rates
  • High RPM engines
  • Turbocharged systems

Trade-offs:

  • Often less filtration precision
  • Shorter service intervals

Common Oil Filter Problems

  • Wrong filter size or thread pitch
  • Double-gasket installation (old gasket stuck)
  • Over-tightening (damages gasket or housing)
  • Cheap filters collapsing internally
  • Bypass valve stuck open

Signs of a Bad or Clogged Filter

  • Low oil pressure warning
  • Engine knocking or ticking
  • Dirty oil immediately after change
  • Oil leaks at filter

Environmental Considerations

  • Spin-on filters retain used oil
  • Must be drained and recycled properly
  • Cartridge filters reduce metal waste

Choosing the Right Oil Filter

Consider:

  1. Manufacturer specifications
  2. Oil change interval
  3. Driving style (short trips vs highway)
  4. Engine type (turbo, high-revving, diesel)
  5. Oil type (synthetic vs conventional)

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