If the transformer is your standard utility transformer, the it probably contains mineral oil. Unless the transformer is going to be huge (over 1000 kva), then it probably contains less than 500 gallons of mineral oil. For fire resistant construction, minimum separation is 5 feet, for non-combustible building, min separation is 15 feet, and for combustible construction the min separation is 25 feet.
This information is from a table in a Cooper Bulletin, and they say the table is a reproduction from the Factory Mutual Guide Loss Prevention Data (LPD) 5-4/14-8.
Factory Mutual - so it is a requirement from an insurance company. Better check with the building owner's insurance to be sure.
The serving utility may have their own standards as well, better check with them.
I can't remember the exact number, but I think the definitions of fire resistant, non-combustible, and combustible construction is located in NFPA 220? I was surprised at how involved it was to determine the type of construction. It goes through each piece of the building assembly and gives requirements on what is considered fire resistant, non-combustible, and combustible.