I got a Mercury 15 HP 1999, I want to change oil of the lower gear unit. But
I am not sure which oil to buy,

It is basically 90w but the oil sold for lower ends is supposed to have more
additives to deal with water intrusion. Since the price is competitive, why not
just buy lower end lub? The various "X" marts usually sell the Lubrimatic and
Penzoil types.