I've heard good things about the internal frame coating.
You could also talk to Krown or another rust-proofing company, they spray basically a commercial version of the eastwood stuff. They're usually relatively gentle on nice cars since all their customers are anal about the appearance of the cars.
If you're sure there isn't ALREADY significant rust, the used oil is a messy and environmentally nasty way to do it. I've done similar jobs with WD40, but never on cars I kept long enough to verify results.