I agree with the 10k mile full syn oil change interval, 6k mile for dino. The 800 engine is easy on oil...heat is very well controlled.
The decision between the taller F800GS or the F700GS really depends on where you want to ride.
Lowering the F800 give you essentially the suspension travel of the F700...still a LOT more than a harley!
I have the F700 predecessor the F650GS. I travel long distances, quite a bit on dirt roads and poor pavement. I rarely go truely off road. This is more than you need to travel alaska, labrador, etc etc. The 19" wheel will give you more stiffness than wire wheels so aggressive riding in paved twisties will be better. I very much like to be able to flat foot my bike...push it back while seated...in general not be a parking lot ballerina tip toeing around.
The F800GS standard height gives you a bit more ground clearance which is important if you will be going over rocks and blow down trees etc. The long travel also help if you plan to jump the bike or go over big wooops. Doing this stuff also really demands that you put some type of knobby tires on for traction in dirt.
The stock "dual sport" tire on both bike are really street tires with just a bit more open tread....no real bite in dirt or mud.
ride both bikes....pick the one that suits you best