Here are two sources of a product that has been used for many years by aircraft manufacturers and home builders of fuselages made of 4130 steel tubing. Might be a good treatment for older bike frames that weren't initially treated internally or need retreating. I don't know what current frame builders use, but I would guess this or similar products they have found to work well.
Aircraft Spruce And Specialty Co.
http://www.aircraftspruce.com/
TUBE SEAL LINE OIL
Prevents rust and corrosion inside aircraft tubing structures. Tubeseal is a blend containing a very fine preservative oil (MIL-L-21260) which has a characteristic of climbing the tube wall and spreading over the entire surface to provide lasting protection. Penetrates and reveals very small pin holes in a weld and in time, due to exposure, congeal and seal the hole. 1 quart will treat 2 light aircraft fuselages. Complete instructions on label.
Description Part No. Price Buy
POLY-FIBER TUBESEAL QUART 03-14000 $17.90 Request Quote
TUBESEAL (LINE OIL) 03-14005 $9.90
WAG-AERO
http://store.wagaero.com/
Tubeseal, quart, FAA Approved
Model:
SKU: K-323-000
Tubeseal, quart, FAA Approved $16.25 Code 4
George Jones Dallas, TX