Check oil pressure with a mechanical gauge. You have to pull the sending unit out anyway to replace it. Put a mechanical gauge(borrow one if you have to) on that port and see what you have. Don't just put the sending unit on and see if the light goes out. 
Agreed on this point.
Better and relatively low cost to investigate first. If you assume wrong the potential cost goes up dramatically.
Borrow and or rent a good gauge to test. If that is not an option go buy a cheap mechanical in dash gauge. Don't even bother mounting it. Just Plumb and test.
Definately go with the higher viscosity oil if it has pressure but measures on the low side.
Good luck,
Mike
