Yes, there is some controversy about this in the industry, and I am not an expert on the legal ramifications. I believe you are correct with regard to fuel sales, but machine sales are likely in a different category.

I base my assumption on this (from IT Law Wiki): A most favored nation clause (also called a most favored customer clause or most favored licensee clause ) is a contract provision in which a seller (or licensor ) agrees to give the buyer (or licensee ) the best terms it makes available to any other buyer (or licensee ).

sample clause: "All of the benefits and terms granted by seller [lessor] [licensor] herein are at least as favorable as the benefits and terms granted by seller [lessor] [licensor] to any previous buyer [lessee] [licensee] of the hardware [software] described in this Agreement. Should seller [lessor] [licensor] enter into any subsequent agreement with any other buyer [lessee] [licensee], [during the term of this Agreement,] which provides for benefits or terms more favorable than those contained in this Agreement, then this Agreement shall be deemed to be modified to provide buyer [lessee] [licensee] with those more favorable benefits and terms.

Seller [lessor] [licensor] shall notify buyer [lessee] [licensee] promptly of the existence of such more favorable benefits and terms and buyer [lessee] [licensee] shall have the right to receive the more favorable benefits and terms immediately. If requested in writing by the buyer [lessee] [licensee], seller [lessor] [licensor] shall amend this Agreement to contain the more favorable terms and conditions."