I don't recall for sure, but it seems like it took more than a quart to fill from empty. If yours only took a 1/4 of a quart, you should be fine. Obviously, there's a small leak somewhere, probably shaft seals. That's why there's oil and grime all over the sight glass and under the gearbox.
Now you know how to check it. You should be opening up that compartment before starting every time, anyway, to check the water level, so keeping the blower gearbox sight glass clear will be one more thing to do.
BTW, nice photo on your initial search for the sight glass. If you hadn't been told there was a sight glass there somewhere, you'd have never known. It looked just like the head of a bolt or plug. That's how many older bus sight glasses are, and the reason many owners don't know how to check the level of the oil in the gearbox. If there are no leaks, the owner/mechanic gets out of the habit of checking (because it's uneventful). Unfortunately, when a leak develops, it's not obvious because the sight glass is crusted over with grime. Don't let yourself get into the habit of ignoring it.
Oh, and JR, Craig's not ALWAYS right, and he'll be the first one to tell you so.