what sort of oily poo? is it soft or is it liquid? if it's liquid, you need to take him to the vet as soon as possible! (i am not exaggerating, i promise) take his nuggets off him and leave just hay.
it would be a good idea to check what food he's been on previously. every change in a chinchilla diet must be done very gradually (usually a month or so) - this is because the bacteria in their gastrointestinal tract takes a long time to adjust to new feed. any upset to the gi system can cause serious problems and can even be fatal.
if the poo is just soft, ie the poo is well formed, you have separate poos but they're a bit squishy or start sticking together, keep him on hay until you find out what his previous food was, then start feeding that and it should hopefully get back to normal. if it's soft to the point where there are no separate poos and it's a thick liquid, you absolutely need to go to the vet today. i suspect the chin is quite young as it's been bought so very little. they can dehydrate and die quite quickly. there are other problems this can cause but i think dehydration is probably the most immediate problem. sorry, i don't mean to scare you!