Is transformer oil suitable for use in welding machine transformers?

Generally, no. Transformer oil is formulated for sealed electrical apparatus and controlled insulation systems, not as a general purpose coolant or lubricant. Using it in improvised ways raises fire, contamination, and health risks, and it may degrade insulation if the system is not designed for oil immersion. Some specialized equipment can be oil cooled by design, but that uses engineered sealing, compatible materials, and safety controls rather than ad hoc filling.