I believe that old "rule of thumb" was specifically geared toward the SBC and was just a general guideline, I would certainly not be using it as a validation mechanism for deviating significantly from spec. Honda went with wider bearings and journals for example when they started going thinner, and this was discussed on here at some length a while back as the Japanese OEM's were the first to really chase the ultra-thin oil thing.
I would never want the following exchange to play-out as a result of advice received here:
Bob: Gee Fred, how'd you toss that rod through the side of the block?
Fred: Well you know Bob, I was on this message board and I was told as long as 0w-16 gave me 10psi per 1,000RPM, I'd be fine, so I put it in and I had 10psi at 1,000RPM so off I went. Engine started knocking after I ran it hard up an on-ramp and then we had a big 'ol cloud of smoke and here we are!
Bob: Doesn't your engine spec like 5w-40?
Fred: Yes, but these boys were really convincing, they really seemed know their stuff!
Bob: Are they going to be paying to fix this?