If you've got springs to suit your weight, then 7.5 would be a good choice if your all up rider weight is around the 15 stone mark. Speaking from personal experience.
If you give Darren, on 01449777161, at MCT, he’ll put you right on what springs you’ll need for what you want to do, and what is the most suitable fork oil weight.
A heavier weight fork oil will increase the damping, make them slower to compress and slower to recover. This does mean that you could end up with a harsher ride because there won't be so much "give" in the system so the suspension won't absorb bumps as easily and could remain compressed if you hit two bumps in a row.The first thing I'd do if I were you would be to look at videos explaining how to check the sag and rebound and see how the bike is reacting with the current oil setup, then you'd have a better idea as to whether to change the oil weight or not.