Here's some info to back up what daTeacha said: From
Daily Fuel Economy Tip » Turn Your Car Into a Hybrid by Simply Turning it Off "So, when should you turn off your engine? Believe it or not, more than 10 seconds of idling uses more fuel than restarting your engine. As a rule of thumb, if youæ±*e going to stop for 10 seconds or more - except in traffic - turn off your engine."
A diesel engine has about double the compression ratio and will likely take about twice the electrical energy to restart it. Therefore, the time to run vs. shut down is about 20 seconds for a diesel.
I doubt that if your engine is fully warmed up that letting it idle is going to cause any fuel to condense and go into the crankcase. However, the cost of energy to run it vs. restart it is the overriding consideration. Safety, though, is another consideration. It's safer to shut it down and put Park brake on and leave in gear when dismounting the tractor. Therefore, when needing to dismount for more that 20 seconds, it's both fuel efficient and safer to shut it down.
By the way, I shut my car engines down at long traffic signals or if stopped for a long delay in traffic.
Ralph