Shell Diala-X (transformer oil) would be the logical choice if you can get it. Arbitrarily filling to a certain point would be ok assuming all dummy loads have identical resistor placement. I would look to be sure that the resistor was at least covered completely by a few inches. Chemical activity, ignition temperature, specific heat, among other things are criteria needed for the proper choice of filling agent. Additives are a really bad idea, typically they have large polar moments, to cause them to stick to metal surfaces as an aid to lubrication, as such any automotive oil is not good, they may eventually attack chemically the resistor and the can. Type F Ford transmission fluid has particles added for friction, since the clutch packs have no material facings. In say a Chrysler product, the clutches and bands have friction linings so these friction agents are not added. The friction agents may sludge out into the bottom of the can after time in the type F fluids, and like I stated all other automotive lubricants may have chemical activity that is detrimental to the resistor. On another site I posted that I put a gallon of vegetable oil in mine in 1988, and the can is still under my bench just as happy as can be, little chemical activity for what it's worth, and no odors. The poster on the other site didn't realize in his rancid oil theory that you first have to cook for a long time with the oil, something not applicable to brand new oil. But in truth even this is not the best choice, the Diala-X is better, and probably pure mineral oil is better for many reasons, but way too expensive in my mind to be worth using. If you cannot buy mineral oil or get the Diala-X, I would use vegetable oil, much cheaper. Never use old or used transformer oil, it is likely to contain polychlorinated biphenyls (PCB's), a super dangerous thing to have around.
