Re: How to check and fill 2003 40hp Yamaha lower end.
I have a 2003 Yamaha 50hp and just did the same thing you're doing. You don't really check the oil level, you just replace the oil. There are two large screws on the lower unit (probably on the right side of the motor if looking at it from the rear). They are both towards the front of the lower unit (closer to the transom than to the prop). One is about a foot above the other. You place a container below the lower unit and remove the bottom screw. Oil will start to flow down the skeg into the container slowly. This is your change to align the container perfectly to catch the oil. Then remove the top screw. Once removed, the oil will flow quickly. It's only approx. 10 ounces if I remember correctly. Let it fully drain, for about 10 minutes or so. Next you want to inspect the oil. If it's cloudy (milky), there is water in it. In this case, you have a leaky seal which will have to be replaced before you refill the lower unit. Replacing the seals (I believe there are three of them in the lower unit) is a topic for another post. Hopefully you don't have any evidence of water in the gearcase to begin with. If the oil smells burnt, you probably had low oil in the gearcase making the gears grind and get hot. Also a possible leak in a seal. Hopefully you don't find this either. If the oil checks out ok, then proceed to refill the gearcase as follows...
Check your manual or look online for what oil Yamaha recommends. My manual called for 90 weight gearcase oil.
To refill the gearcase, attach a pump to the lower of the two holes and pump the oil in until it starts to ooze out of the top hole. Try not to allow any air to be pumped into the gearcase. If there are air bubbles coming out of the top hole, then allow it to sit for a while so that all of the air escapes, then pump in more oil until it oozes out the top hole. You want the gearcase to be full of oil (no air) all the way from its bottom up to the top hole. Once it is full, replace the top screw with a NEW gasket. The gasket is a compressible rubber washer that it not intended for reuse. You are supposed to replace them with each lower unit oil change. Next, remove the oil pump from the lower hole. Quickly replace the bottom screw with a NEW gasket. You will need to do this quickly because some oil will leak out when you remove the pump.
Wipe all excess oil off off the lower unit. After you're done, it's a good idea to turn the prop or run the motor a bit to get the oil worked in. It's a 10 minute job once you've done it before. The hardest part is the cleanup.
