When you change the oil, you might try rigging a flat piece of 10gauge copper wire pounded flat & formed into a sort of "?" mark on the end that'll fit in the oil drain hole, to use as a rake to give you an idea of what kind & level of deposit load may be on the bottom of the oil pan sump.
Any signs of varnish on the engine oil dipstick????
The specified Motorcraft oil filter has very low pressure drop across the filter.
After replacing the IAC
, did you remove the Battery B- cable to wipe the KAM
, which also wipes the fuel trim tables & cold & warm idle strategy, then go through the cold & warm idle relearn routine???? If not, doing the idle relearn routine may help some on idle speed, oil pressure warning light flicker & driveability. EDIT: Idle relearn routine https://www.taurusclub.com/forum/1358165-post4.html
Its a little difficult right now to know if its a deposit load, or bearing wear, or a combination there-of problem, but going to the 5W-40 might shed some light on it if the rattle or idle low oil pressure warning light problems subside, or take a hike.
It's also not known if the vehicle really just has 73K on it, or if it's 173k, or 273K, but if its really just 73K, the engine shouldn't be worn out, or sludged up if its had regular oil change service over its life time & the sump rake test might shed so light on the question.
Seeing as how this is a Vulcan 3.0L, is it making any kind of squeeking sound on top of the engine, back next to the firewall where the distributor used to be????? If so, suspect a problem with the cam sync assy, off which the engine oil pump is driven. This mechanical arrangement is known for poor lube problems & siezure or parts that strip the oil pump drive gears, so have a close listen to that area, maybe with a stethoscope & if any untoward noise is coming from that part of the engine, don't drive it, until that complete assy is changed out with new Motorcraft parts.
More thoughts for consideration, keep us posted on your findings.