I experimented a bit when tantalising fl was worth something.Our sawmill produced quarter sawn larch posts pre the days of treatment.
It was hard work as each round was manually handled.
You could expect 30+ years life from them.
They were a good market for early plantation thinning.
The Risborough Lab tested treated timber for longevity and one of the quickest rotting timber is sycamore. This rose to the top of the longevity list after treatment!
I have some garden furniture that is made from sycamore and that was treated when a flat-pack and this has stood in both my gardens for 40 years to date
The minute the chemical composition was reduced to just food colouring ‘treatment’ became a waste of time in both senses.
Personally I use creosoted round 4”-5” 6’ posts and 7”-8” 8’ strainers from Mcveigh Parker.
My old Parmiter bashes in the posts nicely but I have to drill a 6” pilot hole for the strainers then bash them.
SS
Sycamore i found didn't last as long as you'd like- and is as weak as widdle.
Beech took treatment well, and is strong....still no good for ground contact though.
(I was working in a building today with beech cladding 30 years plus old)
Oak sapwood treats very well, NB sapwood is as strong as heartwood, but only as long as it's fresh.
I was working on raised decking made of just such material, been in situ for mebbe 20 years.