I've been thinking

About this Joe, if there is indeed a pressure drop across the center three mains, it is quite likely caused by extra "leakage" on those three.... those feeds are supplying one main and two rod bearings, whereas the front and rear mains are only supplying one main and one rod bearing. So the "pressure loss" is likely created by the extra flow going to those areas.... more volume used = less pressure present....but is that a problem??
I agree in part with what ZZ28 is saying. I have used a very large main bearing clearances in the past by "accident"... well, a stuff-up actually, that produced a max of around 40psi oil pressure and hardly registering on the gauge at idle. I had to run it and still turned the thing to 8k with tongue-in-cheek.... I pulled the engine down the next weekend expecting the worst but neither rod nor mains had a mark on them.
I think that taught me something, that the pressure is the means by which volume travels thru the engine.... the pressure does not actually create the oil film or contribute to its strength.
In ZZ28's example, the "low" pressure indicates (to me anyway) that despite the rather tight clearances, there is still adequate volume flowing across the bearings .... perhaps as a result of a very light oil being used? Myself, I would not consider running such tight clearances, everything must be spot-on and there is no room for error.