I maintain some equipment at our local rodeo grounds including a 1980s or 90s era water truck with an international diesel. It gets used only when events are coming up which is to say not very often...it might sit for 4 months then be used every day for a week then sit several more months before repeating. The oil might have 50 hours run time on it. The oil has not been changed in at least 2 years and looks like fresh oil out of the bottle. I have serviced many diesels over the years and never have seen this before, there is no hint of soot, the oil is clean...how is this possible?