I think the Wikipedia article on Gear Oils answers most of this.
Essentially gear oils are rated using a "GL" number. The higher the number, the higher pressures the oil can take. Basically there is GL1 through GL6. But GL4 and GL5 are the only "active" ratings currently in use. GL5 rated oils have the most "EP" there is.
en.wikipedia.org
Essentially gear oils are rated using a "GL" number. The higher the number, the higher pressures the oil can take. Basically there is GL1 through GL6. But GL4 and GL5 are the only "active" ratings currently in use. GL5 rated oils have the most "EP" there is.

Gear oil - Wikipedia
