[mattfranklin]

Two issues:
1) Getting the air out, which might be less of a problem if the filter had a check valve (I didn't think Chevy's had them).
2) Changing the filter, which would dump all over the frame when you unscrewed it. And maybe worse with a check valve.

EDIT: The third thing is that when it's on its side you can't pre-fill the filter with oil before your screw it on, so there is a dry start for at least a few seconds.