Be very careful when changing from 7.5W to 15W (read: don't)
While your forks will definitely dive less on the brakes with harder damping you will also make the fork stiff causing the wheel to lift off and just as worse to bottom out on bad roads and dirt roads.
IMHO the original damping of the F650 GS and Dakar is already too hard for bad tarmac and gravel (being set up for high speed stability) causing the bike to feel unsettled and indifferent under rough conditions. This will get even worse with heavier fork oil. Cornering hard and fast on bad tarmac will reveal an overdampened front end immediately by lack of steering precision (difficult to keep the line) and increased kickback tendency (more notable on road bikes with wide front tires)
The only clean solution to get rid of too much nose dive of a bike is getting aftermarket fork springs with more progression.
Pressuirizing the fork (where possible) does work as well as decreasing the size of the air chamber by adding more oil (very carefully, not more than 10 mm in one go). A lot of pressure or a significantly higher oil level will affect the lifetime of your fork seals.
Very important: Fit fork boots if you don't have them already. It's a joke that a bike like the Dakar comes without them.

Re:GSing. It's quite unusual to see Touratech recommending heavier oil for their progressive springs. Most manufacturers go with the original viscosity or recommend even lighter oil to improve the response of the suspension (see yngveer) unless the original setup is seriously underdampened.

For fork oil you can use ATF (Automatic Transmission Fluid) which comes from every car / gearbox workshop and is usually 7.5W. It was filled into motorcycle forks before the advent of specialised fork oils and works fine:-)

Alternatively you can use hydraulic fluid but it is only commonly available in quite heavy viscosities (20W and heavier). Lighter hydraulic fluids like 5W and 10W are available in cold and arctic climates and in the fork lift of the nearest commercial cold storage:-)

Check your inner tubes very carefully for stone dings and smothen them out with a very fine oil stone followed by a nice polish. On young bikes oil leaks are usually caused by stone ding damaged inner tubes.