Need help from a motorhead.

Okay, this is what my dad taught me to do.

See that large standard head screw on your second picture down just above the black intake strainer? That is screw one. There should be a large straight head screw just like the first one towards the front of the cone on the lower unit. It will be on the underside, and might be easier to see with the motor trimmed up. That is screw two.

Okay, run the motor with water cuffs on for a few minutes. This will heat up the gear oil a bit

shut the motor off, trim it down all the way, and towl down the lower unit

put an oil pan under the lower unit.

remove screw #2, wipe it off, being carefull to clean any metal shavings.

now remove screw #1

let all the oil drain out.

now the harder part.

get your tube of oil ready, pop the top of it

get screw #1 in your hand, and keep screw #2 very close.

shove the tip of the oil tube in hole #2 and squeeze!

Squeeze it all in there, being carefull to keep the tube forced right up against the opening.

When the unit is full, oil will start to come out of hole #1.

Here is the hardest part.

Once its full, but with the oil tube still shoved in the hole, screw in screw number one.

now get screw #2 ready

quickly in one motion, pull out the oil tube and put screw #2 in there, screw it in, being carefull to let as little oil out as possible.

Your done!!

Right on DeadFish...
 
Doug- get yourself the whole Pennzoil (or equivalent) kit...  It will make the process a lot cleaner and less messy.
 
Often times, the sheer vaccum will hold the oil in the lower unit until you unscrew "screw #1" and break the tension by venting that opening.
 
The Pennzoil Pump has been good to me, at least.  And the threading of the nozzle can go right into the lower hole on the unit to fill.  It is easier than the old way I used to do it. 
 
Are you running SeaFoam or Sty-bil through the gas lines and stuff?  Thats about all I do for my outboard, but this winter I am pulling the carb so I can rebuild it and ensure it will be sticky-free in April.
 
There is no way I am going to survive another summer without a reliable outboard.  This past summer almost killed me.  But I should be up and cruising at higher speeds and with less stress by the time the ice fades.
 
Outboards take away more of my cash and patience than girlfriends do...  No exaggeration.  Not even close.

That oil pump looks really easy!!! I wish I would have known about them before! ****! Okay Doug, you gotta get one of those. I'm getting one before I winterize the Yammy.

Here's a good reference pic for where the lower screw should be.

Oh yeah Lance- one more thing:
 
You might need an adapter for the Yamalube unless they make their own pump package.
 
I am pretty sure the standard Pennzoil kit (I like Pennzoil) is set up for OMC out of the box.
 
I am sure the adapter is only a few bucks if needed, but the pump is a HUGE help and makes the whole process really quick and easy.  I've even seem vacuum pumps for outboards in FL that mechanically pull out ALL the lube, just like if you were replacing ATF or something in a car.  Might be overkill though unless you are running giant twin (or triple) outboards with a whole lotta fluid.
 
Hook- I bet you won't need more than a quart for your 40 horse.  My 30 doesn't take much at all.
 
Also- take note of the condition of your used lube since it will give you an indication of what's going on inside your motor.  Milky white oil is bad news since it means you are getting water in there due to a bad outer/inner seal or maybe from the vent screw itself.
 
Also (and this is critical), make sure you don't crossthread either slotted screw!!!
 
I did it the first time and nearly ruined everything.  Thread the screw in REVERSE until you feel it set into the threads and then tighten clockwise as usual.  If the lower unite threads get screwed up, it is a pricey fix!