Probably nothing. It depends on what they're recommending and what you're putting in. For instance if they're recommending 10W-30 and you're putting in 10W-40, that's probably fine but if they're recommending 10W-30 and you put in straight 30 weight that would probably not be recommended. some of the newer engines need those specific double viscosity oils. Also they may recamend the new synthetic oils, in which case I would stick to what they say because it may be a reason for them to void your warranty later if something were to happen, even if it weren't related to the oil. If you have a new vehicle it's probably worth a quick call to the local Nappa auto parts store or garage. If it's older I wouldn't worry about it.