A lot of diesels will not make heat unless they are turning 1300 or so RPM. A gas motor will warm up at idle, but a diesel... with all that cast iron.. usually big radiator.. won't make enough heat at idle to burn the fuel efficiently. Not saying race the cold motor, but get some RPM into it after you think the oil is circulating good... I let mine warm up around 1400 plus or minus... I also try to get the tractor moving after about ten minutes, to start warming up the rest.