Pontiacs are like HD motorcyles, they're not leaking, they're marking their territory..........
Having worked on these cars when they were new, to only a few years old ( Co-op in 1970, at the local Pontiac dealer). Plus having worked at GM dealerships such as olds, and buick, which also used rope seals, it was few, and far between to see a completely dry rear main seal on one of these engines, they mostly all seeped after 10,000 miles. People that owned them were told it was inevitable, and was normal. After a few years some of them leaked pretty bad.
My parents had a 63 Bonneville that had low miles on it, that they bought used in 1965. It leaked from the day they bought it, to when it went to the boneyard. It got slowly worse as the years rolled by too.
My father was a mechanic most of his adult life, and knew he would have to pull the engine to put a new rear main seal on it. He was going to do it at one point, then I guess he talked himself out of it. It was kind of unusual, for my dad to put up with something like this, because we had just gotten the driveway paved with asphalt while he owned that car, and new asphalt doesn't bode well with a leaking rear main seal.........
Instead of putting a new rear main in it he bought a 69 GP that didn't do anymore than slightly seep. I bought the 63 from him and used it for a winter car, until some guy hit it during a snowstorm, and I sent it to the boneyard.
So most of them leaked to varying degrees, people just poured more oil in them. Seldom did they get repaired, because the engine had to come out to do the job right, and most places would tell their customers that it may leak worse than it did before they "fixed it". Code for ,"We really don't want the job at all".
While I worked on buicks at a dealership it wasn't really out of the norm for the factory to put a couple of rear main seals in buick engines, and if they couldn't get the rear main sealed to a customers expectation, they would shortblock the car under warranty. I did a couple buicks, and put new shortblocks in them while I worked there. You can look the rear main area over, and see nothing amiss, but they just won't seal sometimes. It would be hard to determine if the rear main grove, and the crank weren't on the exact same axis, making the seal light on one side of the crankshaft, leading to a slight leak. Pontiac blocks when machined for the main seal groove, were all over the place, not a bunch of time was spent in that area of the block.
Just relaying real world experieces here of having worked on these cars when there were hundreds of thousands of them on the roads, from a mechanics standpoint.