I've used penzoil SAE 80W-90 marine gear lube in my 15hp Evinrude 1984 without problems. It can be obtained at Academy, Walmart, and etc. Make sure it is marine gear lube.
Good reading here at this link!
https://www.sschapterpsa.com/ramblings/O ... 20info.htm
The marine Dr. changing gear oil.
https://www.youtube.com/watch?v=Wj0la6c5FDg