Unloaded idle for a long period of time is NOT the way to warm up a diesel, leads to wet stacking and other ills. Let it idle at moderate speed (around half or rated or slightly more) for a minute to get the fluids circulating and then put it under MODERATE load to warm up.
It is the same way you should warm up your gas powered car or pickup, once it starts and the idle settles to near curb idle then it is time to start driving but not at full throttle acceleration. With my Corvette Z06 I wait for heavy acceleration until the oil is up to 175-180 degrees which takes a little time with its 10 quart dry sump design.
My last two GMC Duramax diesel pickups have a great cold start strategy built in with the vehicle starting at normal idle speed and then when ambient is around 32F or lower, slowly ramping up to around 1,300 RPM over a minute or so. At the same time, the variable vane turbocharger goes to the most restrictive exhaust setting and the Allison automatic transmission is commanded into gear with the output shaft locked so that the engine has a decent warmup load on it. It is an excellent warmup strategy.
And with tractors, I think a lot of people are glossing over the difference between full throttle and full load. Just because the engine is turning rated RPM doesn't mean it is operating anywhere near full load and that is a big difference between break-in under a reasonable versus a heavy load. I will be doing the first full mowing with my new Kubota F2690 tomorrow and for the first couple of times I will run it at 90-100% of rated RPM for proper cutting action but I will have the deck height set and use a ground speed that doesn't put the engine under extreme loading for those initial hours.
Rodger