I've been using engine oil as a coolant for high power resistors but am wondering whether something like peanut, sunflower or avocado oil would be better. Engine oil gives off an awful smell when it breaks down, smells like rotten eggs. Can't find transformer oil. Any thoughts?
Yucky things will grow in vegetable oil. May I suggest hydraulic oil? The only additive is anti-foaming stuff. Hydraulic oil can get really, really hot before it begins to break down. You are probably smelling additives in the oil, not the oil itself.
Can your oil move up through the resistors as it gets hot?
The Heath Cantenna just used mineral oil, used to be available by the gallon from the drug store. Handled a kilowatt and was parked in a gallon can, so that might give you some idea of what you might need for volume.
There are various additives that are in engine oil for anti-wear properties, added to the mineral oil base. You aren't going to need those and they're the source of the smell, they contain sulfur. If your project gets that hot, you need more oil!
Vegetable oils generally will go rancid, jell, get wildlife growing in them, absorb water and do other things that mineral oils don't. Unless you're making a turkey fryer, give them a pass.
Transformer oil is just purified mineral oil, moisture-free. Can still be had from the right suppliers. Might have to buy a barrel, the retail market is kind of small.