You want to know the level of the oil as it's going to sit in the motor on the water. If you just level your trailer, or check it on a flat garage floor, you'll be fine. Too much oil isn't good thing for a few reasons. As stated above you can blow oil out anywhere it can get out, making a big mess. If the crank is hitting the oil in the pan, it will whip it into a froth which doesn't pump well, plus it robs horse power. Excessive oil hitting the crank and other moving parts can create even more crankcase pressure than normal which can blow out engine seals like crank seals.
