Part of the confusion is because there is an agricultural exemption (see http://www.state.il.us/osfm/PetroChemSaf/180AbovegroundFuelDispensi...)

Section 180.20 Aboveground Storage - Dispensing
a) Storage of Class I, II, or III liquids (except kerosene) shall be in accordance with 41 Ill. Adm. Code 160 and 41 Ill.
Adm. Code 170, except aboveground storage for dispensing may occur at the following facilities under the following circumstances:
1) Agricultural storage, which is limited to farms, tree nurseries, fish farms, tree farms, sod farms or orchards;

Remember Section 180.20 subsections (a)(1). That is a very important reference.  Here is some info:

Dispensing at facilities specified in subsections (a)(1) and (a)(3) of this Section shall only be in accordance with the following:
1) A top-fill opening shall be provided and shall be equipped with a closure designed so that it may be locked;
2) A vent shall be provided to relieve such vacuum or pressure as will develop in normal operation or fire exposure; such vent shall have a minimum unobstructed opening of one and one-half inches in diameter;
3) Tanks equipped with a permanently connected pumping device shall be equipped with a padlock to prevent tampering; an effective antisiphoning device shall be included in the pump discharge, and siphons or internal pressure discharge devices are prohibited;
4) Tanks elevated for gravity discharge may be designed with an opening in the bottom or end of the tank for gravity dispensing of flammable or combustible liquids and shall be mounted and equipped as follows:
A) Supports to elevate the tank shall be of steel and designed to provide stability;
B) Openings for gravity discharge shall be equipped with an internal safety valve, which will close automatically in the event of fire through the operation of a heat actuated releasing device. The valve shall also be capable of being manually operated. If this valve cannot be operated manually, it shall be supplemented by a second manually operated valve. The gravity discharge outlet shall be provided with an approved hose equipped with a self-closing nozzle at the discharge end and be of a type that can be padlocked to its hanger to prevent tampering;
C) When not being used to dispense fuel, nozzles shall be hung off of the ground on a hanger and shall be padlocked in place to avoid tampering;
5) Tanks for the storage of flammable or combustible liquids shall be marked with the name of the product they contain and "FLAMMABLE -- KEEP FIRE AND FLAME AWAY", both in letters at least four inches high and in contrasting color from the tank on which they are marked;
6) Facilities described in subsection (a)(1) of this Section shall be allowed a maximum of four aboveground tanks per facility of up to 2,500 gallons each for storage of flammable or combustible liquids, provided the total amount of any single type of fuel does not exceed 5,000 gallons, and any such liquid is stored outside buildings in a tank constructed throughout of steel and made vapor tight.
7) Facilities described in subsection (a)(3) of this Section shall be allowed a maximum of two aboveground fuel dispensing tanks per facility up to 2,500 gallons each for storage of flammable or combustible liquids, provided any such liquid is stored in a tank constructed throughout of steel and made vapor tight;
8) Tanks installed or replaced after January 1, 1998 shall be located as required by subsection (b)(10) of this Section.

Did you catch that?  Subsection 6) said "provided the total amount of any single type of fuel does not exceed 5,000 gallons"  Your 8,000 gallon tank throws you out of this exemption and makes you fall under the same rules anyone else has to follow.  I won't copy all of them here, but here are some tidbits:

8) Tanks for the storage of flammable or combustible liquids shall be marked with the name of the product they contain and "FLAMMABLE - KEEP FIRE AND FLAME AWAY", both in letters at least four inches high and in contrasting color from the tank on which they are marked;
9) ... a maximum of two aboveground tanks per facility of up to 2,500 gallons each ...

So if I read it correctly you have to put your 8,000 tank underground.  That opens up a WHOLE bunch of new regs I won't even start to look at here.  Oh, and here is an interesting subsection:

10) Tanks shall be kept outside and at least 30 feet from any building, or combustible or flammable stored liquid, liquid petroleum and shall be so located or such additional distance to buildings shall be provided to insure that no vehicle, equipment or vessel being filled directly from any such tank shall be closer than 30 feet to any building, combustible or flammable stored liquid, liquid petroleum (LP) gas tanks or property lines,

So IF I understand it (which may not be correct) if you are filling  your tractor with a disk and crumbler attached from an aboveground tank and unfold the implements to grease them while you wait there can't be a building within 30 ft of ANY part of anything connected to the tractor being filled.

In my experience the OSFM folks are probably the easiest agency in Illinois to deal with ... as long as you approach them before you do anything and tell them you are doing your best to be compliant with the law.  But if they get the feeling you are trying to pull something ... Lord have mercy on you.