Any treatment will cause some discolouring but I would suggest raw linseed oil, try it first on a spare piece. You will need at least two coats or more for good protection.
I've used Ronseal decking protector(natural finish) on my shed because i wanted it to remain as close in colour to natural wood as possible. i was very pleased with the results. it darkened it very slightly though but the natural appearance is very good.
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