I believe that if you check the GMPowertrain website, the prior-gen Chevy small block is still available for some uses, plus being produced for crate motors for GMGoodwrench and GMParts. In that criteria, the orig 305 in my (purchased new) 1977 Camaro was pulled out at 576K -- the reason? 4 leaking core plugs (i.e., freeze plugs) on the side of the cylinder block. It was still running well and not using massive amounts of oil (the oil? Castrol GTX 20W-50, which later was upgraded to SyntecBlend 20W-50 toward the end). I suspect the oil was coming in through the valve guides and a little from the rings.
When new, it used 1/2 qt in 4K. When usage increased to where it took more than 1 qt to go 5K, then I went to a change interval of 'put one quart in, change it when it needs another one'. As things progressed farther, still, in the winter months there were some times that things went longer than that. End result, it got oil changes when I felt like getting under it rather than by miles per se. Even with that "so-so" oil change schedule, there was no significant accumulation of things in the lifter valley.
As for timing chains/belts, I consider that part of any high mileage engine maintenance situation--I call it "insurance". When I did the cam/intake/carb upgrade at 92K, I put a Cloyes Plus timing chain set in and it is still in the motor (now residing on an engine stand).
I didn't mean for this to be an endorsement for Castrol GTX, but that's what it had in it from the first oil change when new (until far, far later in life). I'm sure that others might have similar experiences with other brands. At that time, too, 20W-50 was a little heavy compared to the normal 10W-30 recommendations. In the coldest weather (i.e., below freezing), when new it had only a second or two that a lifter would tick upon first start in the morning.
Therefore, with the greater accuracy of factory machine work that goes into current production motors, I'd say that most any of them (4 cyl and otherwise) should make 500K with decent maintenance and current motor oils in them (with a quality oil filter). If they might need a timing chain/belt change as part of normal maintenance, I wouldn't count off for that as the basic short block is a greater determiner of longevity, but a broken/failed timing chain/belt CAN affect the short block too, just as no oil in the crankcase would.
I know that Mercedes makes a big deal of their extended-mile club. Chevy's now doing that with their Silverados. I used to get tickled at Toyota television ads about how great it was that their vehicles (by customer testimonials) would easily go 100K miles with no trouble.
I also have observed that many people brag about how their oriental import brand vehicle will last, but I feel that such longevity (of those engines) is highly influenced by a rigorous oil change schedule (as some definitely need it!) that seems to be the mantra of those brands and owners. I suspect that if an American brand vehicle was treated to the same regular maintenance schedule, they would have done equally as well--by observation.
The other thing is that many of these imports were used as daily drivers for high-mileage commutes and they were "beaters" and not traded-in every three years for a new one. I observed as the bodies deteriorated, the interiors were gone in a few years (from the TX sun), and the paint was far beyond any "magic wax" resurrection, yet the engines purred right along.
On the other hand, most USA brand vehicles were traded-in every 3-5 years (well BEFORE 100K!) so not everyone knew that they could last as long as they can (engine/powertrain wise). Some might claim there was a reason for the USA brands being traded that often, but I feel it was more for style than necessity that they were traded--a sign of prosperity, too.
With all of the electronic controls on modern engines, the machining advances, balance shafts, and much better motor oils, not to mention greatly improved designs of vehicle bodies and chassis items, I see no real reason (especially for engines for which 'oil gel' is not a problem with particular motor oils) for any current production vehicle to not be able to see the other side of 500K miles, with decent care and maintenance.
You might also attrribute some of that longevity to overdrive transmissions, but highway cruise rpm in OD is not that far from what the old 3-speed automatics (as in my Camaro) with "highway gears" (also, as in my Camaro) in them would do. The OD automatics generally have deeper "direct" gear ratios which allow better performance and still have lower cruise rpms on the highway--especially for the smaller engines.
Just some thoughts,
CBODY67