Double frame and rust between frame rails

I bought this 2008 International 9900 tractor with double frame rails.

There is rust on the frame on the outside. As far as between frames goes, there is rust between frames, but it is NOT yet pushing the frames apart and it appears to be relatively mild.

I am not worried about rust on the outside, as I spray all trucks with hydraulic oil every few months and it keeps them from rusting. But what about the inside?

I am in Chicago and there is salt in winter and I want to nip this problem in the bud. Is there some realistic way to stop this rust from further developing? I read about installing grease fitting and pumping grease in there. Does it really work well? Should I pump grease or hydraulic oil?

Again this is about BETWEEN double frame rails.

 

What about pumping grease in via grease fittings? Maybe slightly dilute grease with gasoline to make it flow better?

 

The double rails stop around the back of the cab, yes.

I do NOT seek to eliminate rust. I just seek to stop its progression. The way it looks now, is that the integrity of the rails is not yet compromised. But it will be if the rust keeps expanding. So I just want to stop it.

On the OUTSIDE of rails, I know that spraying or wiping the area with hydraulic oil stops rust completely. But I understand that sandwiched between rails, it is trickier.

 

Anyway, here's what I ended up deciding to do.

I decided to drill holes and install grease fittings on the sides of the double rail.

I have a high pressure air/hydraulic pump and I made an adapter to pump hydraulic oil into grease fittings. I have a large quantity of really sticky hydraulic oil that loves clinging to metal surfaces. I will pump a lot of that oil into those grease fittings, repeatedly, so it would diffuse between the frame rails and through the rust between frames and kind of fill the area.

Right now the truck is really dry due to dry hot summer and I want to infuse the double frame with oil repeatedly for a few weeks. I hope that this will fill the frame with oil and greatly slow down the rusting process. Maybe do it once a few months. I will also spray same oil on the outside of the truck frame.

I am a scrapper and have over a thousand gallons of hydraulic oil that I burn in my winter furnace so there is no real cost to the oil.