Coolant flow

and for the record, the make up of the fuel had little to do with the heat produced. For practical purposes the energy produced to overcome a load, be it friction at idle, or towing a trailer is pretty much the same whether its 1970 fuel or the latest RFG.
The difference in compositions mostly effects reid vapor pressure, and distilation curves. For practical purposes this means how easy it is to initiate a flame kernal, how long it takes to grow, and how well it burns up the heavy end.