Transformer oil is commonly called mineral oil (but I think the term mineral oil applies to many other substances as well). If you call a supplier and ask for transformer oil they'll probably know what you want. To my knowledge there's not a standard spec. As I remember one major distributor only had two types available... with or without artificial oxidation inhibitor. But you may want to add specs related to testing of parameters that are important to you. Dielectric strength is a fundamental requirement for electrical equipmet... try asking for 30kv (also have to specify which type of test). Moisture - generally don't want to put >20ppm (by Karl Fisher) into your transformers, so you should specify 20ppm or less. Neutralization number -<0.1 mg/kg k-oh. PCB - not detectable. Typical values for some other parameters that aren't critical may include: 0.88 specific gravity, 57-62 SSU (viscoity), 130-145C flash point, -40C pour point. Also there are 2 varieties of testing you can do: one is a certified test provided by your supplier; the other is a sample you draw from the tanker when it arrives or througout the fille process (provides more direct confirmation).
has a list of transformer-related standards from IEEE.
62-1995 IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus--Part 1: Oil Filled Power Transformers, Regulators, and Reactors - has a few specs for moisture etc.
There's also a few guides in that link for reclamation of tranformer oil, specs for silicon oil (fire retardant). I remember one more specifically on limits for transformer oil (prior to putting it in transformers), but I didnt see it at the above link.
Midel provides their oil up to 50ppm moisture? Way too high for high-voltage transformers.
Doble Engineering has a Tranformer Oil Purchase Specification (TOPS) which can be used when specifing txf oil.
There is also an ASTM Standard D-117 "Standard Method of Testing and Specifications for Electrical Insulating Oils of Petroleum origin"
Transformer oil is also commonly refered to as 10C oil.
