Viscosity is measured by different scales for engine oil and gear oil. The two systems aren't directly comparable, number-for-number. Here's a reasonably accurate comparison between the two systems. Please remember that I said "reasonably accurate", as there is some overlap.
SAE Engine Oil ........ SAE Gear Oil
10w thru mid range of 20w = 75w
Mid 20w thru mid 30w = 80w
Mid 30w thru low 40w = 85w
Low 40w thru 50w = 90w
60w up = 140w
Looking at these ranges we notice that 50w engine oil is the same "real" viscosity as 90w gear oil. So it stands to reason that they might be substituted for each other. But remember that there are other factors involved than just viscosity. The ability to withstand shear, and additives designed for a particular end use are also important.
SAE Engine Oil ........ SAE Gear Oil
10w thru mid range of 20w = 75w
Mid 20w thru mid 30w = 80w
Mid 30w thru low 40w = 85w
Low 40w thru 50w = 90w
60w up = 140w
Looking at these ranges we notice that 50w engine oil is the same "real" viscosity as 90w gear oil. So it stands to reason that they might be substituted for each other. But remember that there are other factors involved than just viscosity. The ability to withstand shear, and additives designed for a particular end use are also important.
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