Around 25 swans, geese and ducks have been rescued so far at Bumble Hole Pool, at Bumble Hole Nature Reserve in Netherton.
The spillage, which happened on December 29, was described by an RSPCA animal rescuer as being the worst he had ever seen.
Now Dudley Council and Severn Trent Water have urged people to come forward with any information they have over the pollution.
RSPCA inspector Boris Lasserre said: "This is the worst oil spill I’ve ever seen. It has affected a lot of the birds very badly, with many of them collapsed and hypothermic.
"But with the help of a fantastic team of local volunteers, we initially managed to save around 25 birds which were sent for treatment at our Stapeley Grange specialist wildlife centre in Nantwich. The rescue operation is sadly ongoing as more and more birds are arriving at the lake and becoming affected by the pollution.
“It is a very difficult rescue operation as many of the birds are almost impossible to reach, not just in the water but stuck in wooded undergrowth around the lake. Sadly, a few of the birds haven’t made it, but the volunteers have been brilliant – without them it would be so much worse."
So far 12 geese, nine ducks, three coots and one swan have been rescued and taken to the RSPCA's Stapeley Grange Wildlife Centre in Nantwich – where the toxic oil was washed off them.
But five of the ducks and one goose were so badly affected they didn't make it. The rest of the birds are being cared for until they are ready to be returned to the wild, once Bumble Hole has been cleared up.
Small fish have also been killed as a result of the spillage, which is believed to be mainly red diesel with traces of cooking oil. It is believed to have come through the surface water drainage system and a culvert into the pool.
Contractors Veolia and Aqua Force are working on behalf of the council and with nature reserve volunteers to contain the diesel and clear it.
Councillor Karen Shakespeare, cabinet member for public realm, said: "I hope whoever has done this is able to sleep at night. It has seen a number of small fish killed and cost the taxpayer thousands to clean up.
"Without the quick actions of volunteers at the nature reserve who reported it, we could have seen an even more serious impact on the nature reserve and our wonderful local wildlife who suffered greatly as a result of this despicable act.
"We are therefore very keen to find whoever is responsible for the spillage and I would urge anyone who may have any information to get in touch.
"It looks like it may have been a deliberate act, and if that is the case and we find whoever is responsible we will not hesitate to prosecute through the courts.
"People need to understand that the substances poured away without a thought for where it ends up can not only kill wildlife but they can also be prosecuted. Pleading ignorance will be no defence.
"This is a lesson to us all to be careful what we are tipping down our drains, as often surface water from homes and the highway feeds directly into natural pools used by our wildlife.
How you can help:
Anyone with information is urged to contact Dudley Council Plus by emailing firstname.lastname@example.org or calling 0300 555 2345.
Anyone who sees pollution on water or land should call the Environment Agency 24-hour incident hotline on 0800 80 70 60.
Concerns for a wild animal that has come into contact with oil or other contaminants should be reported to the RSPCA’s emergency line on 0300 1234 999.