Because you are paying for your oil when you place the order we have to charge you for a specific volume. We are not subsequently allowed to take more money from your card than the amount you have already paid, so unfortunately it is not possible to have your tank topped-up.
While Suppliers acknowledge it may not be possible to determine exactly what your tank will hold they do ask that the order is for a specific volume/litres rounded to the nearest 100 litres - your best estimate of what will go in the tank. If the tank can't take the full volume ordered you will be refunded accordingly.