Wow - 10 years without an oil change. You are lucky that thing even runs. It will take 20 ounces of motor oil. Plus, they put markings for oil level on the dipstick for a reason. You should be fine using SAE30 motor oil. Most likely that is what was in it to begin with, though you can also use 10w-30. You can drain the oil by tipping it and draining form the fill tube which is proably easiest for most people or there may also be a pan drain underneath the deck of the mower. If there is one there, you can use a 3/8" socket wrench with no socket attached to unscrew it.
