Re: 90 hp Mercury lower unit oil
I have that engine. Go to WW and as Chris and wired said buy a quart of Quicksilver/Mercury Premium LU oil in the quart in the package that has the pump. Makes it a nice clean simple job. On the 6 cyl engines remove the vent plug at the top and drain on the bottom. On the 3 cyl engine like mine, there are 2 vent plugs on top. Ensure that you have the washers (mine are blue) under the screws to seal the screws.
Screw the pump adapter into the drain plug and with the one vent engine pump till you have oil coming out and then a little more to reduce the bubbles. Don't have to get them all out. On the 2 vent plugs do the same as before but when the oil starts coming out of the front vent, plug it and continue pumping until it also comes out of the aft plug and cap it.
Then with a cleaned off magnet on the drain screw....mine has it, some don't, remove the pump adapter and insert the screw. Loosing some oil is normal but keep it to a min. Use a big screwdriver to keep from dinging up the screw slots and get them tight using both hands.
Engine should be vertical in the fill/drain processes.
Mark