Just thinking through the theory. Gasoline engines intake a fuel/air mixture at the stoichiometric ratio evenly disbursed, about 14.7:1 or 15 parts air to 1 part fuel. When that vaporized mixture hits cold cylinder walls the gasoline can condense; therefore, an enriched ratio at cold temps ala choke on an older carbureted engine or computer programmed enrichment based on temperatures for a fuel injected engine.
Diesels don't care about the stoichiometric ratio. They always get cylinders full of air and then just enough fuel to keep the engine running at the governed speed whether it is controlled by a mechanical governor or computer. Instead of 14.7:1 a diesel may be running at 150:1 at idle, but this varies from engine design to engine design, will differ more between a direct injection and indirect injection engine.
Newer fuels are cleaner, leave fewer deposits.
But engines do like a more or less constant temperature. In general pistons are an aluminum alloy, blocks and heads are cast iron (for ag tractors), bearings are of differing alloys, etc. When you shut down the engine it begins to cool with the various parts cooling at different rates. Clearances change. When restarted it once again needs to warm so components are at their normal clearance but also realizing the engine is designed to have proper clearances at all working temperatures. So it would make sense to keep the engine at a constant operating temperature and not go through a number of heat/cool cycles.
Conversely even at idle and with an oil film between moving components operating time equals wear. Not much wear at idle but still wear.
Also if you intend to trade in your tractor you will get more money for a low hour machine.
In the real world having had hundreds of test machines operating mixed in customer fleets during a time span, often 2 years, prior to new product introduction I found that a typical large paving contractor will have a person start the engine of every machine in the fleet early in the day prior to all of the other crew members arriving on the job. The engines then run all day even if something in the chain of machines breaks down and everybody has to stop. Some operators do stop their engines, most don't.
After all this I don't believe there is a definitive answer. I know you can hear all kinds of expert opinions but having been involved with engine design and production from starting with a clean sheet of paper to analyzing the results after years of production the best answer I have is you can do whatever you want.
One extreme operation I still remember is a machine in one of my test fleets getting astounding fuel economy. I could tract all the machines in my test fleet from wherever in the world I happened to be and when I plotted daily hours and fuel used this particular machine stood out so I sent one of my guys to check the operation. The contractor was short of operators so they had split operation of 2 machines, a dozer and a vibratory compactor, between one operator. He would start both machines at the beginning of the day, then use the dozer to move material, switch to the compactor to do its job, then back to the dozer never shutting an engine down during the day. Lots of idle time for both machines. When I sold my test machines I didn't get any more money for this one because of its easy life. I sold it based on the hour meter.
Thanks, I will check that out. Might even read the manual.