Which timber

[Cuppa]

Nah i reckon that is Oregon for sure mate the grain is way to stringy for Pine, pine is a broader grain. That second pic is classic Oregon rot. When you say "Milled" do you mean Dressed? or rough sawn?

Dunno, I wouldn't call it dressed, but perhaps that's the correct term - the 'mini corrugated' finish on the two rotten pieces, commonly seen on pine.

I believe there are two sorts of Oregon. Years ago it was a very durable timber which would stand up to weather for many years, but the oregon today is of a similar colour but a totally different timber (?Douglas Fir?). I built some external steps with the stuff & they rotted out within 3 to 4 years.

I also thought that todays treated pine was somewhat health friendlier that the old arsenic based treated pine (blue instead of green).?

I must admit when I saw 'bench', I thought 'work bench'.

For an outdoor 'bench seat' I think your redgum sleeper idea sounds good.