It's also called "hypoid oil" because it's used in hypoid geartrains. It's a very thick, somewhat sticky oil used in things like manual transmissions.
not all 90 wt oil is "hypoid oil" hypoid means that there are additives in the oil to help with shaft driven engines, these additives can damage some clutch components if it is used in the wrong applications, however, if the engine calls for hypoid oil make sure to use it.