The NYPD and FDNY are among the victims of a massive scam that pumped dirty heating oil into thousands of furnaces across the city for years, sources told The Post on Thursday.

Authorities on Wednesday raided five of the city’s largest fuel oil companies, seizing about 50 delivery trucks and other evidence as part of an investigation by the Manhattan District Attorney’s Office and other agencies, sources said.

Castle Oil of The Bronx was one of the companies raided, but sources would not disclose the names of the others.

The multimillion-dollar scheme involved mixing used motor oil, which has little value, with pure heating oil and selling the tainted blend to NYPD police stations, FDNY fire houses, hospitals, apartment buildings and small businesses, authorities said.

State law regulates the disposal of used oil because it is highly toxic when dumped or burned. It contains lead, benzene and other contaminants.

The criminal investigation stems from a civil suit filed nearly two years ago by Manhattan developers against the former Castle Oil company, which last year was bought out by Sprague Energy.

The suit accused the company of buying used motor oil from garages and mixing it with pure heating oil.