Nothing beats oil for older rust prone cars/trucks. I do this in the fall when it's dry...drilling holes if needed...getting it in the rockers/frame channels...door bottoms etc. Sounds like the frame also for these trucks... Use drained ATF.

This won't work where it washes off...like in wheel wells. There I use the alum/plastic/tar/rubber roof repair stuff on a roll...sticks very well on a clean surface...this or roof tar & fabric.