If a creature has at least one oil counter on it, it gets +1/+1 due to Ichorplate Golem's effect. It does not get any additional bonus for having more oil counters on it.

The first ability will check to see if a creature has any oil counters on it at the moment it entered the battlefield. If it doesn't, the ability won't trigger at all. This means that if the creature doesn't enter the battlefield with oil counters on it, there isn't an opportunity to put oil counters on it after it has already entered to get Ichorplate Golem's first ability to trigger. If the ability does trigger, it will check again as it tries to resolve. If the creature has no oil counters on it at that time, the ability will do nothing.