Preserving wood for raised beds

[Philthy]

(Apologies for asking again what must be a very recycled question)

I have some pressure treated gravel boards from Wickes.
I want to use them to edge a couple of beds, so direct ground contact.
I was wondering whether I ought to apply any additional preservative to them, or if I go with them as they are, what the typical timeframe might be before they start deteriorating.
I'm not keen on the idea of lining them on the inside, as I think it'll create hiding places for pests, and the first time I catch the lining with a fork or something the moisture will get in anyway.
The usual considerations of nothing toxic / plant safe, apply.
I've used linseed oil on a chair before, but that isn't outside 24/7.

What do you recommend / any particular brand names?
(I did try the search function but it was a bit inconclusive)

Thanks

[Kristen]

My view is that whatever safety tests have been made on Tanalised timber it was probable in a normally-dry environment, rather than in damp soil in close proximity to root crops which will then be eaten ...

So i took the decision to line mine [I sued damp proof membrane] so that there was a barrier between any chemicals in the wood, and the soil (and also in the hope that it might make the wood last longer too).

[sparrow100]

I have the same boards for mine - I paint mine with cuprinol because I want colours, and I nail weed membrane (which lines the paths) to the insides. No idea how long it will take them to deteriorate. The previous boards on the plot were rotten and had been there for about 5 years, but had been mostly buried in the ground.

I've noticed that people on my site pile soil to the middle of the bed over winter if the bed isn't in use, which I guess is to try and keep the boards a bit drier.