Try adjusting the clearance on a steel-backed bearing with less than 1/16" of soft bearing material sometime. By the time the oil pressure drops, and/or the noise level increases, it's too late to even replace the bearing in most cases- - - -there's also crankshaft wear and/or damage. Also, Babbit can imbed a lot more foreign material without totally destroying the bearing layer.
Since none of the full-pressure stovebolt engines ran Babbit rods, oil pressure really isn't a good indicator of bearing clearance. The pressure-lubed bearings such as main and cam bearings are all thinwall steel backed bearings. Model T and Model A engines are a different breed of cat, with poured and line-bored bearings in those locations.
Jerry
