Oil or Stain Treated Pine Logs in Retaining Wall?

Hi there
Hope I'm posting to the best-fit forum...

I've recently purchased a home in regional NSW, and it comes with a fair few retaining walls constructed of treated pine logs & posts (I take it the term "sleeper" refers to those rectangular in shape?). They've been in place for about 25 years I'd say, and a retaining wall guy recently came round and said I could expect another 7-10 years out of them. In some places they still seem in good knick, in others there's a few that appear rotted/swollen/warped, and I'm getting them replaced.
Given the age of the retaining wall, I'm assuming it's CCA treated pine (is there a way to check?) – though any new logs I purchase probably won't be.

I'm here because I want to achieve 2 outcomes:
(i) preserve the retaining wall so its life is extended as long as possible, and
(ii) alter the appearance so the retaining wall colour becomes as close to black/charcoal as possible.

I've done what feels like a mountain of research, but I still can't determine the best way forward (all I know is I don't want to paint):

1. Do I need to be regularly applying some sort of wood preserver (oil?) to the retaining walls? I've read that the fact they are treated (likely H4) guards them against fungi, for example, but not water, UV, etc.
2. If I go with a water-based stain to colour them black (applying after adequate preparation of the timber), do water-based stains have sufficient preserving properties, or do I need to combine a water-based stain with some sort of preserving oil?
3. Would it just be better then to go with an oil-based stain, on the assumption that an oil-based stain has greater preserving properties? or will even an oil-based stain need to be combined with some sort of preserving product? (oil-based stains apparently can't get as black/charcoal as water-based stains)
4. Given my situation, which is preferable: oil- or water-based stain? I've read that water-based stains last longer (ie. less frequent re-coating) but their colour fades worse than oil-based stains?
(5. My gardener suggested he was going to pressure clean the logs and apply Creosote, but after reading about the hazards of Creosote, and the potential that the logs look faded dark brown, instead of black, and remain oily for some time, I'm not in favour of this, unless I'm missing something?)

Thanks to anyone for their help, I'm not really sure what professional/expert out in the 'real world' I could even turn to..
Cheers

I was about to post something back in the Porters Palm Beach black thread, but you have gone into more detail about your application requirements here.

I would suggest that the future failure of your retaining walls will be due to water penetration at the rear of the wall. The front, that you are looking at colouring black would have minimal deterioration compared to the rear holding back the soil.

The resting wall guy has given a life span of another 7 years or so before replacement so whatever you put on the front isn’t going to be super long term. Even if you need to apply the finish to the front twice in the 7 years it isn’t too bad.

The issue I see with the paint is that it creates a film over the timber, which you will only be doing from one side. When it starts to break down or blister, you will likely need to strip it back and reapply. Applying a stain doesn’t create a film, so when time comes to reapply it’s just a clean and reapplication.

Your choice of application on the front is unlikely to either extend or reduce to any significant extent the life of your retaining wall. If I was in your situation I would look for a product that gives you the appearance you are after with minimal maintenance- I don’t want to come across as a fan boy, but I would at least do a small test with the Palm Beach (or other similar) and see what you think.

I used cabbots exterior decking stain on some rough sawn sleepers.... but this was purely for looks.

really the side that needs treating is the side you can't access, the one with all the dirt and water up against it. I wouldn't bother doing anything untill you're ready to replace the whole wall.

Years ago an old bloke at a now gone timber yard was very direct when I asked a similar question on treated pine logs as a pergola.
The Aussie sun and elements will dry the time out so you have to oil it regularly, yes at least annually.
Over the past 40 years I've followed his advice and regularly oiled the treated pine with clear natural timber oil, decking oil, and he was for sure right!