Too much oil cannot build significantly more pressure than too little oil. I
f the breather tube is plugged, it'll build up pressure whether you have too much, too little, or no oil at all.
The breather tube is located near the top of one of the axle tubes (or on some axles, at the top of the housing). If you're putting enough gear oil in such that it comes out the breather tube like an overflow line, the diff would have to have both axle tubes nearly full, which would require at least three or four gallons of oil. Furthermore, at the point at which it would blow out the breather from being filled from the fill plug, the oil would have to be at least a couple of inches higher than the fill plug... difficult to do to say the least.
Too much oil has nothing to do with the seal's integrity. The seals don't care how much oil there is in the housing. They only care whether or not there's pressure behind them, and if there is, that's the fault of the breather. The fact that oil gets forced out past them when the breather is plugged has nothing to do with the level.
Now, if you want to talk about specific volumes, say you add the precise 2.8 quarts of fluid (or whatever volume your particular axle calls for). How do you know how much fluid was left in the axle? What was caught by the bearings? What volume was coating the internal parts? How do you measure that? Or do you drain the axle, clean the internals piece by piece, reassemble it, and then fill the precise volume without spilling a drop?
It's an axle. It doesn't care that you're an ounce or two over the recommended fill volume. It's an axle with bearings and gears, not a Swiss watch!
} make sure you fill it the same way you did. It's fine.
All you do is squirt the gear oil into the fill hole until it drips out. The vent tube is not an overflow, but a vent to prevent pressure build up within the axle housing...
