Choosing Engine Oil: Viscosity, Specifications and Approvals Explained

Oil labels, decoded

One grade. Two parts. Three checks.

0W-20 — example only, not a recommendation

0W: Low-temperature cranking and pumping requirements

20: Viscosity requirements at high-temperature test conditions

These are grades, not outdoor-temperature limits.

Match all applicable requirements in your manual

Correct viscosity
Match the complete grade.
Performance specifications
Match every required standard.
Manufacturer approval
Verify the exact product and grade if an approval or license is required.

“Full synthetic” alone does not establish any of these matches.

Original Roadfelt diagram. Based on API viscosity definitions, the API Motor Oil Guide and GM’s licensing list.

Two bottles can carry the same large viscosity number and still be intended for different engines. Choosing oil starts with the requirements for your exact car, then checking whether a particular product satisfies all of them.

Keep three questions separate: Is this the right viscosity? Does it meet the required performance specification? Does it carry any manufacturer approval or license the car requires? Price and brand come after those checks.

Start with the right manual

Identify the model year, engine and market before searching the oil shelf. If you are unsure which engine you have, use our guide to identifying your car's exact version.

Find the engine-oil section, capacities and specifications, and scheduled-maintenance instructions. Copy the requirements into a note rather than relying on memory. Read the footnotes too: an alternative permitted for a particular temperature or emergency top-up is not automatically the normal service recommendation.

Record the specified viscosity, performance standard, any manufacturer specification or approval, and the applicable service schedule. If an online selector contradicts your manual, ask the manufacturer or a qualified workshop to resolve the difference before buying. API directs owners to the vehicle manual before its oil-category charts.

Viscosity describes flow, not overall quality

Consider 0W-20, purely as a label-reading example—not a recommendation for your car.

Label element	What it tells you
0W	The oil meets that grade's low-temperature cranking and pumping requirements.
20	It meets that grade's viscosity requirements at specified high-temperature test conditions.
Both together	A multigrade classification; it does not identify every performance standard or approval.

API's viscosity definitions describe these physical properties. The numbers are not outdoor-temperature limits or a ranking of oil quality. Matching “20” alone is insufficient, and a higher second number is not automatically a better choice.

Use the complete grade specified for your application. Do not switch viscosity solely because the engine is older, the weather is warmer, or another driver uses it. Any alternative should come from applicable manufacturer guidance.

Read the performance standard separately

API and ILSAC specifications address performance beyond viscosity. They are examples of performance standards; copy any other specification in your manual just as precisely. As checked in September 2026, API's Motor Oil Guide includes API SQ and ILSAC GF-7A/GF-7B, introduced in 2025.

For gasoline engines, newer API service categories include the performance requirements of earlier categories. That does not override the required viscosity or a separate automaker specification. API identifies GF-7A as the replacement where GF-6A was recommended; GF-7B replaces GF-6B and applies only to 0W-16. Do not interpret that as permission to substitute 0W-16 for another grade.

Diesel specifications need separate checking. API FA-4 is not interchangeable with CK-4 and several earlier diesel categories; manufacturer compatibility instructions matter. If a diesel manual specifies a different standard, use that exact requirement rather than translating it into a gasoline category yourself.

Check manufacturer approvals precisely

An automaker's oil requirement can add another condition. Where it operates an approval or licensing system, verify the exact product and viscosity in its official list.

For example, GM's dexos1 Gen 3 list identifies the product, supplier, specification, viscosity and license number. Match the relevant fields; a familiar brand name alone is insufficient. This is an example of how to verify a license, not advice to use dexos1 oil in every vehicle.

Treat “recommended for,” “suitable for” and “meets requirements” as claims to check, not automatic proof that the vehicle manufacturer granted approval. Equally, not every manufacturer's specification uses a formal approval list. Follow the evidence and wording required for your car.

“Full synthetic” does not answer those questions. A product may use that description while carrying a different viscosity or specification from the one you need. Compare the complete label and current product data sheet.

Choose the interval from the car's instructions

Selecting the right oil does not establish how long it should remain in the engine. Follow the applicable time, mileage, operating-condition and oil-life-monitor instructions. Do not extend an interval simply because a bottle advertises extended protection.

Ford's guidance illustrates why this is vehicle-specific: some vehicles calculate oil life from their use, while the applicable manual provides the maintenance requirements. Its 2022 Transit instructions also impose a maximum interval and describe a fallback if the monitor fails. Those are instructions for that vehicle, not a universal schedule.

Our maintenance-schedule guide helps turn those instructions into a record you can follow.

Take this checklist to the store or workshop

Photograph the relevant manual page and note your engine and model year.
Match the entire viscosity grade and required performance specification.
Verify any required approval or license for that exact product and grade.
Ask the workshop to identify the oil on its written estimate. If “synthetic oil” is the only description, request more detail before authorizing service; our estimate guide explains what to ask.
Keep the receipt, date, mileage and oil details with your service history.

An oil-pressure warning is a different problem

An oil-change reminder concerns maintenance timing. An oil-pressure warning with the engine running can require immediate action: stop somewhere safe, switch off the engine, and follow the vehicle's warning instructions. Ford explicitly warns against continuing even when the oil level is correct. Do not treat it as a routine reminder or assume that buying a different oil will resolve it.