how to clean an old steel toolbox?

You don't want to eliminate ALL the Petroleum, you want to remove the CRUD, That dark black,gritty, slimy, disgusting, skin staining, thick nastiness that covers the bottom of a toolbox that has never been cleaned properly or not cleaned in a very long time. This advice works with all metal surfaces whether working with woodworking tools or mechanics tools. In my professional opinion (55 years of age, a mechanic with 32 years experience.) you do not use soap and water unless you are working on something to display in a show, but that is an alt other different process that requires EXPENSIVE cleaners and preservatives that are not green and require proper disposal. For our purpose we only need a few simple components.

Nytrile Gloves, in your size!
Rags
a large oil drain pan, 13 quart.
1 empty, 1 gallon can.
1 Gallon, Diesel Fuel
1 quart, WD40 (I buy it by the gallon!)
1 quart Synthetic Oil (Amsoil. Royal Purple, Mobile one!)
1 large container CLR 'Jell' (in a squeeze bottle is best!)
1 LARGE empty coffee can (Metal)
1 semi stiff bristled brush (a parts cleaning brush is best. a firm bristled large paint brush works but takes extra effort!)

PUT ON THE GLOVES, crud causes Cancer, trust me! I have had the skin tumors removed, 32 years, REMEMBER!
Place on end of the toolbox in the 13 quart pan, place other end on bricks so it is stable. Pour about a cup of diesel into the toolbox, use the brush to scrub through the crud. Pour the crud mixture, out, into the pan. use a few rags to wipe out the toolbox. Then wipe off the outside of the toolbox, YES there will be a film of oil on the surfaces. If you do not like the diesel smell, pour equal amounts of Synth. Oil and WD40 into a spray bottle. Spray the mixture every where you want to protect the surface from rust (Or eliminate the diesel smell!) Use clean rags and wipe down again, WALLA, nice clean sweet smelling toolbox. Empty the pan into the coffee can, a week later, do not slosh the coffee can, slowly pour the diesel back into your can OR diesel, 90% of the crud will stay IN the coffee can. Mark the diesel container as follows, "DIrty Diesel, for cleaning!" Now for your tools!
Clean your tools the same way, if you find surface rust, wipe until as dry as you can, place in the empty 13 quart drain pan, use the CLR Jell and the brush, do not get aggressive, just move the solution around until the rust comes off. Use a rag, wipe off the CLR, clean with Diesel again and/or the WD40 mix. If your tool is stiff with rust, place the tool in the empty coffee can, pour in enough WD40 mix and soak until it works freely again, allow the WD40 mix to sit, undisturbed, for a week, repeat pouring BACK into the WD40 mix container.
On woodworking tools, rust and oxidation, use the CLR ONLY! If you use the WD40 mix to free-up a rusted tool, THAT is when you use soap and water to clean the tool afterwards, use a hair dryer to quickly dry the tool.
TOOL Storage, after the cleaning. Saturate a clean rag with cleanest WD40 mix, wring-out the excess, spread the rag out on the bottom of the tool box, place your tools in the toolbox. Before storing for a long period of time. Give a quick spray of WD40 over the open toolbox, close immediately and store. Woodworking tools. Place the tools into your toolbox, cover with five layers of newspaper. Place ONE WD40 Mix Moistened rag, spread out flat, over the newspaper, store your toolbox. NO this is not how you store woodworking tools if you have a Woodworking shop, but if you store your WW tools in your garage and only use them occasionally, this IS how you store metal tools so they remain clean, dry and rust free!
I hope this dispels a great number of arguments about how best to store tools and maintain toolboxes, MY Bid Box is 19 years old and still functions like new, I have air tools that are more than ten years old and never wear out, WHY!?! Because every time I use them I Lube and or oil them before I use them. Then Clean them before putting away and I am talking about tools that are use six days a week, they are my livelihood and I have a collection that appraised at just under $500,000, they fit in THREE toolboxes. My Big Boy is 6' tall X 8'8" long and 32" deep, That MACSIMISER weighs 7800 ponds when full. I have to Off-load half the contents just to ROLL it and one side cabinet is so full I had to custom make a frame under it and use an extra caster to support it. Fortunately I only have to move it avery two year to clean under it, I keep my area THAT clean!

If you remove all the grease, you _may_ actually want to put some back. Spreading a thin layer of oil on tools is one way to help keep them from rusting. But the key word there is "thin" -- about as much as you might use to grease a pan before baking is plenty.

So the right answer may be to clean it up, then to take a paper towel lightly moistened with a thin oil and rub it down.

(Of course if the paint is intact, it too provides protection so you might not need this.)

I recently rescued an old mechanic's toolchest -- the type with a drop front and lots of drawers -- and my approach was to first go over it with WD40 as a cleanser and water repellent, then fill it with packets of silica gel and leave it for a weak so those could absorb any remaining moisture. So far, so good.

10 Wipe up all you can with rags.
20 Scrub with hot water and plenty of soap.
30 Rinse well
40 Dry quickly (hand-held hair dryer works well).
50 If necessary, GOTO 20.
60 Done.

You can try an engine cleaner, like Engine Brite. Spray some inside the toolbox, wait a few minutes and then wipe or rinse, whichever way you choose.

I'd recommend an orange-based hand cleaner or degreaser.  It works wonders on both hands and objects!  Toss a bit in there with some water, scrub it around with a sponge, then rinse.  Repeat if necessary.

a little gasoline and steel wool or wire brush will remove all but the most stubborn gunk (dispose of the resulting grunge appropriately) finish up with dish detergent and hot water

Believe it or not waterless hand cleaner of the type used in the garage is great for cleaning old grease off metal.  Put some on a rag and rub it in then use paper towels to wipe off the residue.  It works by liquefying the grease so it's easy to remove.  It's also save to use since it's non flammable.

It's not save to use on wood since the wood will just soak up more of the grease.

You can get surprisingly aggressive cleaners in the nearest store. Floor cleaners like "Flash" , used neat, will shift heavy grease in minutes - I've cleaned engines with them, and we use them at work to prepare steel for painting.

Steve

Scrape and wipe off as much grease and oil as you can and then wash it using very hot water, a stiff scrub brush and laundry detergent.  Rinse well and dry with rags or paper towels and a blow dryer to dry the small gaps between welds and crimps.
 You can even take it to a coin operated car wash to do the initial cleaning if you like.

If it's that bad, use a pressure washer. I doubt it's really necessary though. Detergent, water and effort are probably all you need.