Oil & Fluids
The cheapest thing you put in a car, and the one that ends the most engines when it goes wrong. This is the part of the site we take most seriously. The specifications on the back of the bottle decide far more than the number on the front.
- Does Coolant Evaporate? The Water Goes, the Antifreeze Stays
- Transmission Fluid Color: Dark Is Normal and Milky Isn't
- Coolant Stop Leak: When It Works and What It Wrecks
- Antifreeze vs Coolant: Are They the Same Thing?
- How to Check Transmission Fluid Level: Dipstick or Sealed
- Check Transmission Fluid Without a Dipstick: 3 Jobs
- What Coolant Does My Car Need? Find the Specification
- Do High-Mileage Cars Burn Oil? Is a Quart per 1,000 Normal
- Synthetic vs Conventional Oil: Most "Synthetic" Is Crude
- What Is Oil Dilution? Fuel in Your Oil, and the Limits
- Can You Mix Synthetic and Conventional Oil? Two Questions
- Wet Belt Engine Oil: Which Spec Your Engine Needs
- 0W-20 Substitute: 5W-20 and Almost Nothing Else
- 0W-20 vs 5W-30: What Happens If You Use the Wrong One
- How to Read an Oil Bottle: API, ILSAC, OEM Approvals
- Can You Mix Coolant Colors? The Color Isn't the Answer
How these oil and fluid guides are written
These guides cover engine oil specifications, viscosity grades and fluid compatibility: what the codes on a bottle mean, what a manufacturer approval is, what happens if you use the wrong oil, and whether fluids can be mixed. The answer turns on the specification, which is the part most pages leave out.
Where the numbers come from
We check specification codes against published references: API's licensing categories, ACEA sequences, and manufacturer approval lists such as dexos1 and WSS-M2C948-B.
We name the test method wherever it matters, ASTM D5800 for NOACK volatility and ASTM D6922 for miscibility, so you can check a claim rather than take it on trust.
What we don't do
We don't test oil. No page here reports a measurement we made, and we recommend no brand, because we have run no comparison that would justify one.
Where we can't trace a widely repeated figure back to a study we could read, the guide says so and points you somewhere instead of giving a number.
Why the manual outranks us
Manufacturers revise and supersede approvals quietly. Your owner's manual and the maker's current approved-oils list are the authority for your engine, and every guide here says so.
These pages explain what the codes mean. They don't replace the code your engine's maker specified.
Found a figure here that doesn't match what you were quoted, or think something on this page is wrong? Tell us. Corrections get read first and the page gets dated when it changes. If you want to know how these guides are put together before you trust one, that's written down too.