Scotty, I have a 2000 7.3L Diesel Excursion with only 158K miles! I only used it occasionally for the 12 years I've owned it, but now it's my daily driver. I love just about everything about it and I've never had a engine problem except a water pump replacement about 4 years ago due to a leak, but I find that when it's 50 or below the oil temperature heats up very slowly and never gets above 130F around town.  I verified this by the car gauge, Scangauge II device, and an IR temp reader. Using it to tow an RV across country this past summer, it did get temps well into the 200's. I am thinking the thermostat may be stuck open.  My questions are:  Is there an easy way to determine if the thermostat is stuck open without removing it?  Also, when operating continuously at this temperature, is this both harmful for engine wear AND inefficient, or just inefficient?  Just one other side note, just before the cooler weather set in here in NC, I replaced my standard 15W40 with 5W40 full synthetic oil. Thanks, and I love your channel!