is this the right oil pressure sending unit for my engine? it seems newish relatively speaking compared to other things in truck. however the wiring seems to be a quick fix. The light just went on. there is no noise from internals that indicate something is wrong. maybe that doesn't matter. can the sending unit switch be faulty and is that what i replace first to see if the problem is resolved.
here are a couple pics of truck and parts in question. any help would be appreciated. maybe an engine rebuild is in order.

 

Wow, what a great looking truck, good to see the side mount spare tire carrier. I had to replace the rear fenders, fortunately kept the intended driver's side fender and spare tire carrier.

That sending unit appears to be correct for the oil pressure light. As mentioned, the connection is a relatively 'quick fix', appears to be 12-14 gauge wire, if going with an inline connection I tend prefer using the appropriate size 12-14 gauge butt joint connector, once the wires are soldiered I like to use 'shrink wrap' to cover the joint. If decide to go this route, suggest test the 90 degree connector before soldiering joint for continuity; if flawed, try cleaning the connector, hopefully still good I have not been able to find replacements, purchased a few aftermarket type 90 degree connectors only to find the female diameter is of a smaller diameter than the standard early model sending unit male connection.

 

That is not the correct oil pressure switch this one is.
https://www.napaonline.com/en/p/ECHOP6281
No need to butcher the factory harness with a butt splice.

 

that switch is triggered at 3.0-8.0 psi? That seems very low. What pressure should I have in my block? And if I bypass the 90 degree connector and touch the wire to ground and the light comes on, should I assume that the switch is working because the light is on?

 

Mathew is correct; although sensor in pic is for an oil pressure light it possibly is correct for later application, appears prior owner swapped the locking spade type connector for a 90 degree female connector usually found on the 'oil pressure' gauge, 2 different types of sensors.

 

A good rule of thumb is no less than 1 PSI for every 100 RPM in Gasoline engines. Or no less than 10 PSI at idle.

So the switch is right in line with minimums.
Really all the idiot light does it tell you your engine is now likely toast.

The Oil pressure switch is a NC (normally closed) switch When it has pressure it is open. If you touch the lead to the oil light to ground the light in the dash should come on.

 

The original spade connector on the white wire with red tracer is for the sender that goes to your oil gauge. The small sender pictured, no matter what tip, is for the idiot light only. If you have a 66 then you should have an oil gauge. Now I have a 65 which had the light but found a correct 66 dash, gauge and wire so that I have both now. The picture below shows the large can for the gauge and the small sender for the light.

 

found the right sender unit and the problem seems to be tackled.

 

sorry this is the photo, i uploaded the wrong one

 

The OP's '66 F100 has the correct sending unit, because...

1956/66 idiot light oil sending unit (B6AZ-9278-B/Motorcraft SW-667) has a stud that the wire connector presses onto.

The idiot light oil sending unit with the clip on wire was introduced in 1967 (C7AZ-9278-B/Motorcraft SW-669).

Both are stamped #6 and are 1/4" -18 pipe thread.

 

Beg to differ on this one B6AZ-9278-B has the spade terminal connector

NOS B6AZ-9278-B Oil Pressure switch 1967 68 Ford Mustang with tachometer | eBay


C7AZ-9278-B has the stud terminal

NOS 67 FORD 390 OIL PRESSURE SWITCH W/ TACH MOTORCRAFT SW-669 C7AZ-9278-B | eBay


His wiring harness clearly had the male spade terminal for the early idiot light oil pressure switch and the dash clearly has an idiot light.

 

in this photo you can see the one on the left is what was in my block when i bought it. worked until i drove it a few hundred miles when the light came on. i removed it and thats when i noticed the right angle push on connector was wired to and taped on to the spade connector that seems to be stock on the truck as far as i can tell. all the parts stores are telling me i should have the one on the right in the photo with the little button on top and use a spade connector. makes sense since thats what is on my wiring harness.
i bought the one on the right and the light seems to stay off. but because of this thread and peace of mind i bought an oil pressure test gauge from pittsburgh. On cold start up the gauge went to 58psi. i drove for 20minutes and pressure stayed between 58 and 62psi. thats through every gear and top speed of 75mph. when i got home and parked the truck i let it run and the psi came down to about 45 and stayed there. So i can assume that the pressure is fine and the newer sending unit is the proper one. I definitely do not have a gauge in my dash as you can see in the photos.
however the new unit does hit the block as you can see in the photos. its thread length is considerably shorter then the other one that failed. i am leaving the gauge in for the foreseeable future just to make sure. then i will put the button style newer unit back in and call it a day.

 

The oil pressure switch that was in the vehicle would have worked, the issue whether problem was with the connection or bad switch. Now you have solve the issue by installing the oil sending unit and connector correct for your application, no need to over think the matter.

If I have not already done so, let me take the opportunity to welcome U to FTE!! Looking forward to your input, do not hesitate should you have any questions, great majority of FTE member's are very knowledgeable and helpful.

 

Ya I was wondering about that as it looked like it was a little short. The one you need to fix the length/height issue is a C5AZ-9278-A / SW 424 as seen here it listed for 65 and 66 Galaxie with the 240 I6

NOS 1965 1966 Ford Galaxie Oil Pressure Switch C5AZ-9278-A | eBay

And here

NOS oil pressure switch Ford products 1954-1969 see description | eBay

Unfortunately no one seems to aftermarket this exact oil pressure switch and uses the B6AZ-9278-B in it's stead or does this.

https://www.rockauto.com/en/moreinfo...435555&jsn=436

Alternatively you could use this guy

https://www.napaonline.com/en/p/MPEOP6664SB

Ya I know sacrilege Chevy parts on a Ford, just don't tell anyone.