There are at least five ways to grade a lubricant, and the numbers do not match across them. A local comparison row can show why an SAE 90 gear oil sits near the same ISO VG band as an SAE 50 engine oil, but that is not a product substitution approval. This guide explains the scales, the source boundaries, and why current standards, product data sheets, additive package, compatibility, equipment requirements, and qualified review still control lubricant selection.
ISO VG: the 40 C Anchor
ISO VG (viscosity grade, ISO 3448) is useful because it anchors an oil row to a 40 C kinematic-viscosity class. The app shows local ISO VG bands from light instrument oils through heavy gear rows, but exact row reconciliation still belongs to the current licensed standard and the product data sheet. ISO VG helps compare viscosity; it does not validate additive package, base stock, approvals, compatibility, or equipment suitability.
Lubricant Grade Decoder
Cross-reference lubricant grades across ISO VG, AGMA, SAE engine, SAE gear, and NLGI. Type any one grade and get the approximate equivalents on the other scales, the kinematic viscosity at 40 C, and the field-truth warning that engine and gear SAE numbers are different scales (SAE 90 gear is about an SAE 50 engine oil). NLGI grease consistency is reported on its own axis with ASTM D217 penetration ranges.
The SAE Engine-vs-Gear Trap
SAE publishes separate viscosity scales for engine oils under J300 and automotive driveline lubricants under J306. They are not the same scale. The local comparison row puts SAE 90 gear oil near ISO VG 220, about the same band as SAE 50 engine oil, but that only explains the numbering trap. For a real product, verify the current SAE thresholds, product data sheet viscosity, viscosity index, cold and high-temperature properties, and OEM requirement.
AGMA Gear Numbers
AGMA 9005 provides industrial gear lubrication classification and application context. The app shows local AGMA-to-ISO comparison rows such as AGMA 5 near ISO VG 220, but those rows are not a gearbox approval. EP and R&O labels can share viscosity while carrying different additive packages, and the gear manufacturer may require a specific performance category, synthetic chemistry, seal compatibility, filtration, operating temperature, or oil-analysis program.
NLGI Is a Different Axis
Grease is not graded by oil viscosity. The NLGI number uses worked penetration context from ASTM D217/NLGI references to describe consistency, or how firm the grease is. The base oil inside the grease has its own viscosity, and the thickener, additive package, compatibility, pumpability, temperature, speed, load, purge path, and OEM requirements are separate checks. You cannot convert an NLGI number to an ISO VG row.