[Bibio]

heavier oil will not stop dive. oil is for dampening, springs are for weight and the size of orifice in the damper rod dictates the flow of the oil (dampening effect). simple damper rod forks will always dive unless you fit some sort of device that has a 'popper' valve such as emulators. now having said that the correct springs and oil will go a long way in helping the situation. but the oil viscosity is a trial and error thing to get right.

emulators work by restricting the flow of the oil till a certain velocity (bump) which then opens a sprung valve to allow more flow of oil. the valve should stay shut for 'slow speed' velocity movements such as braking but then open for 'high speed' things such as bumps.

dont confuse high speed and low speed for speed of bike as in MPH, its the speed of the stanchion movement in the fork tube.

EDIT:
heavier oil restricts the speed of the stanchion so you end up with a harsh ride.

suspension is about keeping the tyre in contact with the road while also keeping the chassis for unwanted movement. its a very difficult thing to get right and everyone has a different 'feel' to how they like suspension.

personally i would suggest changing your braking habits, if your front end is diving when your braking then your braking too fierce. its a v-twin learn to use the engine braking. i mostly use my brakes for stopping or scrubbing speed and not for braking into corners or such (unless i go in a bit hot then i'll trail brake).