theres two ways to go about this.
1, leave it alone and act like its not there

like you said its running fine now
2, complete tear down and acid bath for about 3 times. block and head. (that's how long it took to clean my blacktop head)
what ever you do DON'T be tempted to try and clean it out your self, or you'll block up the oil pickup and fuk the motor.
this happens because in japan. they never service there cheap runarounds. they have so much faith in the quality of the toyota name that they assume it will never die. so just top up the oil when needed.
also the blacktop suffers from the sludge build up worse then any other motor ever made. (if not serviced)