I found a web site with the mainteance manual, mine is on the boat. It lists monitoring the exhaust colour and if it is black is says you are overloading the engine and this should be avoided as it will shorten engine life.
Diesels like to be worked but within limits. It also says if you are running for long periods of time at slower RPMs that every 2 hours you should rev the engine up several times to clear it out.
Web site can be found here