Base oil is bought on a name and delivered as a property table, and the gap between those two things is where most disputes in lubricant supply originate. SN500 is not a specification. It is a commercial shorthand for a viscosity band, and within that band two refineries can produce material that behaves differently enough in a finished blend to require re-validation of the formulation.
The five values that decide suitability
A refinery certificate of analysis carries a dozen or more parameters. For a blender evaluating whether a parcel fits an existing formulation, five of them do almost all the work.
| Parameter | Why it matters | Watch for |
|---|---|---|
| Viscosity at 40 °C and 100 °C | Determines where the oil sits in your blend calculation | Both values, not just one — the ratio implies the VI |
| Viscosity index | Governs how viscosity changes with temperature | Below 95 on a Group I stock signals a weaker cut |
| Flash point (COC) | Safety, volatility and evidence of light-end contamination | A low flash point on a heavy grade suggests blending |
| Pour point | Cold-flow behaviour of the finished product | Seasonal and regional requirements differ sharply |
| Colour (ASTM D1500) | Proxy for refining severity and oxidation state | Darkening between shipments indicates a source change |
The value most buyers skip
Total acid number. It is routinely present on the certificate and routinely ignored, yet a rising TAN is the clearest available signal that material has been stored badly, has oxidised, or is not the virgin product it is represented as. On virgin Group I stock it should sit at or below 0.03 mg KOH/g. A parcel arriving at 0.10 is telling you something about its history that the grade name is not.
A grade name describes what the material was meant to be. The certificate describes what it is.
Source consistency is worth more than a small discount
For a blender running a validated formulation, the largest hidden cost in base oil procurement is not price per tonne — it is the laboratory and production time consumed by re-validating a recipe because the incoming stock came from a different refinery than the last parcel.
This is why we hold single-source allocation for contracted blenders rather than filling each order from whatever is cheapest that week. A two percent saving on a parcel that forces a reformulation is not a saving.
Practical checklist before you accept an offer
- Request the actual certificate, not a typical-values datasheet — they are not the same document.
- Confirm the refinery of origin by name, and ask whether it will be the same on the next parcel.
- Check viscosity at both 40 °C and 100 °C and calculate the implied VI yourself.
- Read the TAN. Query anything above 0.03 mg KOH/g on virgin material.
- Ask whether load-port sampling with a retained sample is available. If the answer is no, ask why.