Oil for welding transformer tank

[WRCC]

Hi Gents, any ideas of a suitable oil to top up the level in my very old (but very good) arc welder.
This was the sort of welder on wheels that was always around a maintainance shop. It would run on 240 or 440V and had a large handle reminicent of an old tramcar to control the current.
The gasket at the top of the tank has gone and some of the oil has been lost. I should like to top it up without danger of strange reactions.

Thanks, Ed

The oil we worked on was a Shell Diala Grade, available at £500 for 200litres. Not sure of availability in smaller packs.
Bazz