ISO VG vs SAE is a common point of confusion when a purchasing list includes both industrial and automotive oils. The numbers come from different grading systems. Before comparing two products, identify the system on each label and the full requirement in the equipment manual.
This guide explains what to keep on your inquiry line and where a viscosity comparison stops being a product match.
1. Identify the viscosity grading system
Write the grade exactly as supplied by the equipment maker. ISO VG is widely used for industrial liquid lubricants. SAE grades appear on engine oils and automotive gear oils, but those two uses refer to different standards.
| System | Where you may see it | Example label |
|---|---|---|
| ISO viscosity grade | Industrial liquid lubricants | ISO VG 46 |
| SAE J300 | Engine oils | SAE 15W-40 |
| SAE J306 | Automotive gear lubricants | SAE 85W-90 |
ISO 3448 sets out the industrial viscosity classification. SAE J300 covers engine oil viscosity, while SAE J306 covers automotive gear lubricants. These are separate classification systems, so the printed numbers are not one common ranking.
2. Keep the ISO VG number with the oil type
An ISO viscosity grade describes a viscosity band at 40°C. It does not tell you the complete oil type or all of the performance needed by the machine. Keep the application and required fluid classification alongside the number.
For example, “ISO VG 46” alone is a short viscosity description. “Anti-wear hydraulic oil, ISO VG 46, with the following equipment requirement…” gives the supplier a clearer starting point. Add the exact standard or manual wording rather than assuming that every product with 46 on the label is suitable.
As another example, an industrial gear oil and a hydraulic oil can carry the same ISO VG number. That shared grade does not establish that either product is a replacement for the other. Compare the intended use and the full performance requirement as well.
3. Read the full SAE grade and application
Keep both parts of a multigrade designation, such as 15W-40 or 85W-90. Do not shorten them to “40 oil” or “90 oil” in an inquiry. You would lose information and make it harder to identify which product family you mean.
Engine oil grades and automotive gear oil grades belong in separate columns on a buying list. A gear oil number of 90 does not mean it is simply a heavier version of an engine oil numbered 40. The relevant SAE classifications assess viscosity under their own rules.
For engine oils, also retain the required performance category and any equipment maker requirement. API’s oil category guidance separates performance categories and directs users back to the owner’s manual.
4. Use comparison charts as a starting point
A chart can help explain the ISO VG vs SAE question, but it does not prove that two oils are interchangeable. Temperature matters, and a viscosity comparison leaves other product requirements unresolved.
The STLE hydraulic lubricant guide discusses viscosity at operating temperature, cold starting and fluid performance together. For a real substitution, take those conditions and the equipment maker’s limits to your technical team.
A useful purchasing rule: do not turn “similar viscosity” into “approved replacement” on a purchase order. Record the proposed product and request the supporting technical review first.
5. Check the TDS before approving a match
Ask for a technical data sheet for the exact proposed grade. Compare viscosity values only when the units and test temperatures match. Then review the fluid type, intended use and performance statements against your requirement.
If a supplier proposes a different designation, ask them to explain the change in writing. Keep the original requirement beside the proposal so your engineer can see what is the same and what needs a decision. Our equipment-based selection guide helps you collect that information.
6. Write a complete grade on the RFQ
For an ISO VG vs SAE discussion, the most useful next step is a clear line item. Include the application, grading system, full grade, required performance, pack size and quantity. Add the current oil name if you are reviewing a change.
- Which machine and lubrication point is this oil for?
- What full grade and performance requirement does the manual give?
- Which proposed changes still need the technical team’s approval?
Use separate lines for hydraulic oils, engine oils and gear and transmission fluids. If you need help comparing a proposed product, send the equipment requirement with your inquiry. Keep the full grade attached to that discussion from the first email through to the order.
