I would suggest using either Dilunet or Aquanett from Owatrol.
Follow the instructions then once the process has ben completed allow to dry then apply a water based Aquadecks for b best results.
You will certainly have to top this oil up every year…
Some oils are not really that good for exterior use… You should always follow the instructions or ask a Decking pro to do it for you.
Hope this is of some help for you