It'* maxing out my gauge. It does have a short body compared to the old one, and it'* part number is 25535479, the only ACDelco part listed by Rock Auto.

Is it possibly the wrong part? Should the battery be disconnected to recognize the signal? Or is it just bad out of the box?

I don't think so. The connector plugged right in. Although it was a bit of a stretch. My wiring was just barely long enough to reach the shorter bodied sensor. Yes, 4 pin.

I checked to make sure, and the needle does swing like it should at key on. It settles on 0 if I don't start the engine, and max if I do. So it looks like I DID NOT damage the wiring pulling on it to get a little extra reach.

How much oil do I stand to lose if I pull it without pulling the filter?

I broke the one on my 95 when I pulled the axle and had to get a new one. I didn't loose more than a quart that I can remember. This was just 3 weeks ago.

Well I undid the battery for a few minutes then took it for a drive to see what would happen. Same thing, but now the PSI is maxed with the engine not running instead of 0.

I just changed mine on the PA a couple weeks ago. I lost maybe a few ounces of oil. I took the old sensor out, plugged it the hole with my finger and then stuck the new one in as quick as possible.

I agree it sounds like you got a bad new sender.