Look for a drain plug on the side of the leg. Tilt the engine to the trailer
position , rotate the motor until the drain plug is facing down, remove
drain plug.
That's the process for my 60hp Merc, should be pretty close for your 90hp.
There has been a lot of discussion about whether the engine manufacturer's
branded oil is better than independent makers. My manual recommends Mercury
or Quicksilver 10W-30, but says that you can use any API certified oil that
meets any or all of these specifications: SH,SG,SF,CF-4,CE,CD,or CDII.

"A.Z" wrote in message
om...
The owners' manual for my 2001 Mercury 4-stroke (90) does not indicate
how to drain the crankcase oil. How do you do that?

Is is really important to use Mercury Quicksilver oil as opposed to
Valvoline or other quality products?