i just drained the oil tank and engine of my 400ex and put a quart of reg 10-40 oil and a quart of fully synthetic 10-40 oil in it. would mixing these two oils cause any harm to the motor? i noticed that when i started my bike after the oil change it smoked some grey smoke when i hit like 3/4 throttle.

 

Long answer short; no.

 

Does it still smoke? Smoke on start up only can be worn valve seals.

 

it only smoked when i was warming it up and when i opened it up past 1/2 throttle. not while it was just idling or at all while i was riding.