I've allways been told when you are changeing the lower unit oil it doesn't matter if the motor is straight up and down or tilted. Now that I have a place to ask dumb questions I would like to see what other people think.

It does matter.
It should be straight up and down which would put the prop spline horizontal. You could tilt it up some to see if any more fluid runs out after you have drained it in the up and down position, but be sure to return it to up and down before you refill.

You can put too much oil in the lower unit. Lower units need room for oil expansion.

Okay, so how much do you fill it? and what is the best way?

I have 02 250 ox66 Yamaha?

Remove bottome plug and top plug. Drain fluid, replace washers on plugs, and refill from the bootom till fluid comes out the top. This pushes the air out filling from the bottom. Have the bottom plug ready, pull out fill tube when oil comes out of top hole and put the plug in bottom. You will lose a little, but not enough to hurt anything. Good luck

Thats the way I do her,......

Dang, that hurt my ribs! Breeeeath.....

I have two holes at the top. One says vent and the other says oil level. The third is the drain at the bottom. Do I still fill from the bottom (with oil level open and vent hole closed) since I have two holes at the top or should I replace the drain plug (with new seals)and fill from the hole next to the oil level hole (top) until the air bubbles come out? 1995 115 Mariner. Thanks in advance.

If you'll replace the top plug after filling BEFORE taking the fill tube out, you'll loose less oil during the swap of the fill tube and bottom plug. Most time I dodn't loose any.