There are at least five ways to grade a lubricant, and the numbers do not match across them. That is how a perfectly good mechanic decides an SAE 90 gear oil must be heavier than an SAE 50 engine oil, when in fact they are about the same. This guide explains the scales and the one habit that keeps you out of trouble: convert through viscosity, not through grade numbers.
ISO VG: the Honest Scale
ISO VG (viscosity grade, ISO 3448) is the easiest to reason about because the number is the actual kinematic viscosity. ISO VG 68 means a kinematic viscosity of 68 cSt at 40 C, give or take 10 percent (61.2 to 74.8). The standard grades step in a roughly 1.5x progression: 32, 46, 68, 100, 150, 220, 320, 460, 680. When you need to compare two oils, convert both to ISO VG and you are comparing the real thing.
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 two separate viscosity scales: one for engine (crankcase) oils under J300, and one for gear oils under J306. They are not the same scale. An SAE 90 gear oil is roughly ISO VG 220, which is about an SAE 50 engine oil. The gear number is bigger only because the gear scale is numbered differently, not because the oil is thicker. Never compare an SAE engine number to an SAE gear number directly.
AGMA Gear Numbers
AGMA 9005 numbers the industrial gear oils 1 through 8, and they track ISO VG closely: AGMA 1 is about ISO VG 46, AGMA 2 is ISO VG 68, on up to AGMA 8 near ISO VG 680. EP (extreme pressure) and R&O (rust and oxidation) variants share the viscosity but carry different additive packages, so match both the viscosity grade and the additive type the gearbox calls for.
NLGI Is a Different Axis
Grease is not graded by viscosity at all. The NLGI number (ASTM D217 cone penetration) measures consistency, how firm the grease is. NLGI 000 is nearly fluid; NLGI 2 is the general-purpose grade most bearings take; NLGI 6 is a hard block. The base oil inside the grease has its own viscosity, which is a separate spec. You cannot convert an NLGI number to an ISO VG; they measure different things.