Oil is a pretty good octane smasher. A little leakage down a valve guide or two and it will rattle like a 6 foot diamondback. Viscosity can affect leakage rates. It can also take up excess clearance, but it VERY OFTEN does more harm than good. Generally, you don't have all your clearances widened by just the right amount, so something gets underoiled.
90W gear oil isn't all that much more viscous than 20W crankcase oil (but it has a wildly wrong additive package).
Straight weights are appropriate for vehicles that never get very cold. Straight "W" weights might appropriate for vehicles that never warm up....at least in really cold temperatures.
IMO, you DON'T run heavier weights in old engines. If the oil pressure is really that bad, it needs a rebuild (or a valve job or a gasket) and you're just going to push it over the edge. Underoiling risks catastrophic damage such as thrown rods. But most likely, it will just hog out all the clearances and make the oil pressure that much worse.
The most important thing is to use the oil the vehicle was designed for. Your old Ford may predate multigrades...
FYI, I have always used factory spec oil in all my vehicles. The only one that has suffered an engine failure was the former Bronco II (2.9L), which made it to 250K miles before it hosed a camshaft. The current owner reported the crank and rod bearings looked "bright, shiny, and new."