The answer to this question is a definite no- You cannot fill your lower unit oil by pouring it in from the top, although that seems easier if you have never done this before.
The lower unit of a boat’s outboard motor has two fill/drain holes: one on the top and one on the bottom.
If you try filling the lower unit with oil from the top, the oil will simply spill over. This is because oil is thick, and because it is thick, it traps the air inside your empty lower unit as you pour.
With nowhere for the air to go, there’s nowhere for the oil to go but all over the ground.
On the other hand, if you try to fill the lower unit with the hole at the bottom open, the oil will enter the top fill hole and immediately come back out.
The proper way to fill your lower unit with oil
Whether you’re winterizing, de-winterizing, or just performing maintenance on your lower unit, it’s crucial to make sure that the lower unit oil is at the correct level and in good condition.
So, what is the proper way to fill the oil in a boat’s lower unit? The answer is simple – you need to leave both holes open and pump the oil in from the bottom. As the oil starts to come out of the top hole, you can then close the top hole while leaving the pump attached.
Once the top fill hole is closed you can detatch the pump from the drain hole.
Yes, some oil will come out, but if you work quickly and have the plug ready to insert once the pump is removed, the spillage will be minimal as there is no way for air to get in with the top-fill hole closed.
This process may sound complicated, but it’s actually quite easy to understand and carry out. The video above will be super helpful if this is your first time changing the oil in your lower unit.