PAG vs PAO Gear Oil: Three Checks Before a Changeover
Compare PAG and PAO gear oil by exact grade, temperature-qualified viscosity and changeover evidence;matching ISO VG alone does not approve replacement.
A PAG gear oil and a PAO gear oil can carry the same ISO viscosity grade without being interchangeable. Before replacing one with the other, separate three questions: does the viscosity suit the duty, can the actual formulations coexist, and does the equipment maker accept the proposed fluid and changeover? Passing one check does not pass the other two.
Start with the exact fluid, not the word synthetic
PAG means polyalkylene glycol; PAO means polyalphaolefin. “Synthetic” therefore describes more than one base-fluid family. The TotalEnergies industrial selection table distinguishes NEVASTANE SY as PAG and NEVASTANE XSH as PAO. MOS lists NEVASTANE SY as a separate offered family, not as another name for XSH.
Use that distinction to identify the documents to compare, not to choose a universal winner. Record the complete product name, viscosity grade and current TDS/SDS revision. If a general web description conflicts with a grade-specific document, resolve the identity before selecting a replacement. Neither an old catalogue entry nor a family name confirms a current certification, batch result or approval for your gearbox.
Read two temperatures before calling the oils equivalent
The MOS catalogue provides a useful bounded example. Both entries below are the named 220 grade; the values are catalogue references under ISO 3104, not measurements of the batch you will receive.
| Catalogue reference | NEVASTANE SY 220 | NEVASTANE XSH 220 |
|---|---|---|
| Kinematic viscosity at 40°C | 220 mm²/s | 220 mm²/s |
| Kinematic viscosity at 100°C | 41.9 mm²/s | 26.0 mm²/s |
The common 40°C value does not make their temperature–viscosity behaviour identical. Conversely, the higher 100°C value does not, by itself, prove lower wear, lower energy use or a better operating temperature. The actual tooth contact, bearing requirements and operating range still govern the choice. Do not extrapolate these two points into an unverified working-temperature limit.
For the PAO family, use the separately identified NEVASTANE XSH product. Keep each grade's figures attached to its own document; do not apply the 220 example to every SY or XSH grade.
Separate miscibility from service compatibility
TotalEnergies' mixing guidance warns against assuming compatibility between lubricant families. Its incompatibility explanation also distinguishes base-fluid separation from additive-related problems. This calls for a product-specific decision, rather than an automatic rule based only on the letters PAG or PAO.
A useful review keeps three evidence levels separate. First, identify whether the two exact fluids remain physically compatible under the supplier's applicable conditions. Second, establish whether the complete formulations retain the required properties when residual old fluid is present. Third, check the equipment's lubricant requirement, seals, coatings and changeover conditions. A visually homogeneous sample addresses only a limited observation; it cannot certify all three levels.
Where the supplier or OEM requires compatibility work, agree the relevant materials, residual-fluid basis, conditions and acceptance criteria before testing. Keep the two fresh fluids as separate references. An unplanned jar mixture is not a validated changeover procedure and should not be used to prescribe a flushing oil, number of rinses or drain interval.
Resolve the changeover decision before ordering
Send MOS the gearbox model and duty, existing full lubricant name, required grade or specification, relevant operating conditions and the current supplier/OEM instruction. Include the quantity, packaging and destination, and request the exact offered grade's current documents and an agreed sample evaluation where needed. This turns a PAG-versus-PAO enquiry into a technically reviewable offer without treating either family as an automatic substitute.