Considering there are multiple standards (that use different scales) for designating viscosity, a comparative viscosity chart can help determine if two or more lubricants have similar viscosities.
But how do you read a gear oil viscosity chart?

Just read it horizontally. For example, an SAE 60 motor oil has a similar viscosity to an SAE 90 gear oil, an AGMA 6 gear lubricant and an ISO 320 hydraulic fluid/gear lubricant. The corresponding kinematic viscosity and Saybolt viscosity are also referenced on the chart.

Kinematic Viscosity

Commonly seen on a lubricant’s data sheet, kinematic viscosity describes a fluid’s visible tendency to flow.
Think of this as the time it takes to watch a fluid pour out of a container.
This tendency to flow is expressed in units suggesting the volume of flow over time, called centistokes (cSt). Kinematic viscosity is usually tested at both 40°C and 100°C.

Saybolt Viscosity

Although centistokes are the most common unit of measurement when determining kinematic viscosity, results may also be reported in Saybolt Universal Seconds (SUS). Viscosity reported in SUS is becoming increasingly rare, but you may still come across it when reading lubricant product information.
Saybolt viscosity is usually tested at both 100°F and 210°F.

Comparative Gear Oil Viscosity Chart