Oil thins when it gets hot and thickens when it gets cold. You want it to stay thick enough to be effective when hot, but you also want it to be thin enough to flow when it's cold.
The #1 thing to look for is the API service rating, (such as "API GL-5"). Once you are sure the oil meets or exceeds that requirement, hot viscosity is the next thing to look at and that's the second of the two numbers (90 in your case). The last thing to look for is the cold weather viscosity rating which has a "W" suffix. You need to make sure the lower unit's required cold viscosity is greater than or equal to the oil's rated cold viscosity. Thus, 80W-90 is a perfectly acceptable oil to use in a lower unit that requires 85W-90. It will just work a little better in cold weather.
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