ive never knowen oil to build up as such, though it can make your hair greasy and weighed down. same curlies can leave it in, like me, others have to wash it out, same can just go with washing a bit out.... everyone's different so yes it is an experiment thing.
if the next day you still have shine and deffinition from the oil, i would use less, or use a low poo instead of cowashing. does it weigh your hair down? maybe you just need to use less.