Should I use natural decking oil on a dark wooden deck?
If you want to see what it will look like with the oil finish applied, clean an inconspicuous area and put a bit of it on. Alternatively, untinted wood finishes often give the same appearance as the look when the wood is wet; clean an area and get it wet for a reasonable facsimile.

I would use what I had in hand anyway, because free is good, and natural weathering/graying will occur soon and your color choice can be changed next season (or the year after) if you prefer a finish with a dark tint.