The oil won;t affect how 'soft' your supension is (your spring - coil or air - determines that).
Oil will affect your fork's damping and lubrication (plus ride height to a small degree if you over or under fill it).
Are you following the guides for the damping oil and the semi-bath lubrication oil (the 2 different weights recommended)?
If so, the 10 weight will probably be fine for both.
I re-did my Rebas with 10 weight in both (not the recommendation) and they're fine.
I'd suggest just doing it and seeing how they feel.
You won't damage anything, but the damping range might be out a little.
On the other hand, if I were you and you haven't opened the oil (and you're not in urgent need) you might as well return it and get the proper weights.
Then there's no question.