I've treated a steel frame before with a spray can of gun oil I got from Wal mart for like 3 bucks. I did it myself BEFORE I built it up. If it has already been built you could probably get away with taking off the stem and seat tube and spraying it in that way. Find all the vent holes and spray it in them. Turn the frame every which way to coat the inside of the tubes you should be good for quite a long time. Be careful of getting a lot of oil down in you BB if it is old style with open bearings. If it is cartridge bearing you should be OK.
Just be careful to wipe all the excess off for a few weeks that will weep out of the vent holes and could drip on braking surfaces like rims.