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).