Hello halfa busa. If the recommended intervals for coolant change are being adhered to then a coolant flush should not be needed. But if you are changing your coolant to another type, away from the manufacturer recommended one, (but to one that is still suitable for your engine) then a flush will be needed. If for example, the manufacturers recommended coolant has silicates in it, then you MUST do a coolant flush if switching to one that does not have them. It's a good idea to do one anyway as you dont' want two potentially different types of coolant mixing. And there is always some of the old stuff left in your engine after draining. And if it has any silicate in it, it will usually result in severe engine damage as the tiny amounts of silicate still left in the engine, and the new coolant mix and react with each other causing massive corrosion inside your engine. Having said that, there are fewer coolants now with silicate in them. But you must check. Then there is the other type of coolant 'flush'. This is one designed to unclogg your radiator and water ways. These are quite vicious to your system and should not be used routinely, only when and if needed.
Regarding your dirty oil thing. If the oil is showing very dirty right after a change is sounds as if you may not be getting all the old oil out. Does your manual tell you to drain the bike on it's centre or side stand? This is important. If you drain it on the wrong stand, you may be leaving a quantity of old dirty oil in the sump which is the culprit. Even a small amount of old oil will have that effect. An easy error to make! However, depending how old and how many miles you have done you could easily flush the engine with a proprietory flushing oil or additive. But I think firstly I would not use a flushing oil or additive in the first instance. I would simply drain the old oil, leave the old filter in place and refill with some cheap oil of the correct grade. Then ride the bike for 10 miles and after letting it cool a little, just change the oil again (and filter this time) using the best oil you can afford. I bet it stays clean for a lot longer after that.
OIL FLUSHING DANGER: If you use a proprietory oil flushing agent to do the task you may get more than you bargained for. Some additives are very aggressive and will remove all old debris and sludge and muck. If the engine has covered substantial mileage, you may then find some oil leaks from crankshaft seals etc as the flushing agent will have removed the plug of muck around the seal. This is not too uncommon and is best to avoid using the flushing additives on higher mileage engines unless absolutely necessary.
