Motorcycle Questions? Ask a Mechanic.
Have you checked the breather tube to the air box for obstructions?
Being overfilled with oil can cause frothy oil. Try a lower oil level.
Water in the oil can also cause frothy looking oil. Change the oil.
Bogging at high speed can be caused by an exhaust pipe or muffler restriction. Check the spark plug color for clues on the fuel mixture.
Black or sooty plug is too much fuel possibly from a plugged exhaust
not allowing the air to flow through the engine. Check the pipe etc.
Check the air filter condition.
Some pressure is normal if the dipstick is opened while running.