About those sending units... if you or anyone else wants to replace them instead of getting a mechanical gauge, keep this in mind... the replacements they sell down at the local parts stores are not so great. About 3 years ago, I had to replace the one in my 77 F150. It was leaking. Pulled it out, and it was the original FoMoCo oil sender. Been in the 4 decades. Went to Oreilly's, got a replacement, put it in, and all was good in the universe again. Until.... a year later I am driving home from Houston and the oil light starts to flicker while going down the highway. Seeing an oil light at 70 mph will certainly make your sphincter tighten up! I didn't have anywhere safe to stop and continued about another mile looking for a good place to pull over and then it came on solid and wouldn't go off. Now I am starting to sweat as I finally get off on an exit. The engine never made any funny noises or ran poorly. Checked the oil and the level was right where it should be. Being it was another 60 miles before I would get home, I called a tow and had it brought home. A 350 dollar tow was certainly cheaper than a few grand for an engine.
The next morning as I'm out there poking around, I consider the oil sender and then decide out of all the possibilities, this one would be the least expensive... and off to the store I go. Come home, put a new sender in it, started the truck, and the light went out as soon as the engine came on! I was really relieved... until...
A year or so later, I thought I saw the oil light come one for just a moment. It was a bright day, so maybe it was just my imagination. Later that night, it came on for a moment and went off. Now it's dark and I know this wasn't my imagination! Checked the oil and it was fine. The next day, the light took an unusually long time to extinguish after starting the engine. I thought, "Really? Another sending unit? Can't be!" Well, it could, and it was! THANK YOU CHINA!! I got the Ford number for a Ford sender and went to my local dealership. The guy said they didn't make it anymore, but there was one sitting on a shelf in the next county and he would have it the next day.... and he did. It was 70 dollars. I put it in and haven't had any issues since then.
I've found more often than not, I am deeply disappointed with parts I buy from the parts houses. Orielly's, Advance, Auto Zone... it doesn't matter which company. So now, for my truck and my old Charger, I try to find original FoMoCo or Mopar parts on ebay or other places on the internet. Depending on the part you're looking for, it might be wise to seek out an original factory piece.
A mechanical gauge is always best. In my Duster, I found a brass tee that I was about to mount both the original idiot light sender AND a fitting for a mechanical gauge.
As far as getting the old one out, using the penetrating oil it likely your best bet... and get that special socket someone showed earlier. I have one... made life really easy for me on the truck... especially considering how many times I had to do that job!