i have started a LOT of fires on cars where oil is used for rust proofing. if any type of welding is done it will burn.
our shop has actually refused to repair cars where oil is used for rust proofing.


i have used 3M cavity wax rust proofing with excellent results for a LONG time. it's a I-Car recommended product in their corrosion training classes.
3 M cavity wax rustproofing

it is wax based so when it gets warm is seeps into the area between the panels and helps seal it up. it also has corrosion inhibitors to prevent rust from forming and prevent the spread of existing rust.