How to Read a Base Oil COA — The Nine Numbers That Decide If the Cargo Is What You Paid For

TDS vs COA — know which document you are holding

A supplier who sends only a TDS has not told you what is in the tank. Accept or reject cargo against the signed specification and the batch COA, not against a marketing typical value.

1. Kinematic viscosity at 40°C and 100°C

These two figures anchor the grade. They determine flow behavior, oil-film thickness and the base stock's position inside an ISO VG or formulation target. Confirm the test method (commonly ASTM D445 / ISO 3104) and compare both results with the contractual tolerance.

2. Viscosity Index (VI)

VI describes how much viscosity changes with temperature. A higher VI means the oil thins less as temperature rises. Because VI is calculated from the two viscosity measurements, an internally inconsistent VI is a reason to query the lab result before accepting the grade claim.

3. Pour point

Pour point is the lowest test temperature at which the oil still flows. It is especially relevant for winter storage, cold-start conditions and long-distance transport through cooler climates. Wax content and dewaxing severity strongly influence the result.

4. Flash point

Flash point is a safety and volatility indicator. An unexpectedly low result can indicate light-end contamination or an off-target distillation cut and may change storage or transport handling requirements. Always compare the test method used with the method specified in the contract.

5. Sulfur

Sulfur is both a quality parameter and part of API base-oil group classification. A batch represented as Group II should meet the relevant sulfur and saturates requirements; a grade name by itself does not override the measured chemistry.

6. Saturates

Saturates, together with sulfur and VI, are central to the API Group I/II/III distinction. If a cargo is sold as Group II or III, the COA should support the classification rather than relying on the supplier's label.

7. Colour

Colour is not a direct performance metric, but an unexpected shift versus the normal grade can flag contamination, oxidation or inconsistent processing. Treat a material colour change as a prompt for investigation rather than as proof by itself.

8. Noack volatility

Noack measures evaporative loss under high-temperature conditions and matters most when the base oil is intended for engine-oil formulations. A higher-than-expected result can affect oil consumption, thickening and formulation margins.

9. Total Acid Number (TAN)

TAN measures acidic constituents. An unexpectedly high TAN can point to oxidation or contamination. For storage-sensitive or long-voyage cargoes, comparing load-port and discharge results can be useful.

Read the COA as a set, not nine isolated numbers

The strongest review is cross-consistency. Viscosity and VI should make sense together. Sulfur, saturates and VI should support the claimed API group. Colour or TAN changes should be considered alongside water, storage history and transport conditions. One unusual value may be a test issue; several unusual values pointing in the same direction are a stronger quality signal.

What to put in the purchase specification

• Grade and intended application.
• Numeric acceptance limits, not only typical values.
• Named test methods or acceptable equivalents.
• Load-port CoQ / COA requirement.
• Independent inspection where commercially justified.
• Clear rejection or off-spec treatment.

FAQ

Can two oils with the same grade name have different COAs? Yes. Different refiners and batches can vary materially inside a broad grade family. Verify substitution against the actual COA.

Should I re-test on arrival? For term or high-value cargoes, yes. Discharge testing is particularly useful for water, TAN and contamination-sensitive parameters.