[Yorky]

Can anyone recommend a wood preservative that only needs to be applied once? I've been using Lidl water based stain which is a nice colour but it needs redoing evry year or two at the most.

Would an oil-based one be more suitable?

[Liparis]

You will be hard pushed to find a preservative you don't have to put on that doesn't need re-doing. An oil-based one might give you a few more years, but you might have to do it again after year three. The more expensive treatments are usually best. The trouble with staining preservative is both the preservative and the staining are being compromised by trying to do two jobs at once, usually biased towards the cosmetics.
Bill.