I don't have any systems anywhere near that old, but I'd be looking at that min_free_kbytes. For comparison, this 8 Gig F16 laptop has 67584.
Have a read of this
(it's on wayback, so could take a while to load the page).
If your systems get really
low, there may not be any room to allocate necessary for the oom-killer itself without it fighting with kswapd. Might explain your observations.
Dicking with the vm tunables is very much a black art - pick a system that is expendable to play on.