This engine has never had the head off. It is a 2004 GM-90 6/1. It only has a half dozen puffs of smoke when cold and none after that. The running characteristics of the oil are indistinguishable from running on diesel. I do run some Sea Foam through it every 10 tanks or so. Water injection also helps with the carbon. I suspect that the main reason it doesn't carbon up badly is that the oil is distilled. I only get 50%-75% recovery from WMO. If you run straight filtered motor oil the engine slobbers. With the distilled oil the heavy fractions, carbon, and dirt form the bottoms of the process and those I either burn in a modified wood stove in the winter or dispose of them at the collection station. Oil from cars has some gas and/or diesel dilution. This oil runs well in the engine. I run the still at a temperature that recovers the lighter fractions. A fellow came by a couple of weeks ago and just HAD to have the still, so I sold it, knowing full well that I can build a better one. That one had a 5 gallon capacity. The new one I'm building will be a continuous distiller running on full automatic with a PLC. A still can also be built that is wood fired but I like the electric version with its accurate thermocouple temperature control. I heat the oil to around 150 degrees and let it sit a couple of hours. Any water collects on the bottom and is blown off before the distillation process starts. Once the water is drained the temp is increased to the flash point of the particular oil and once the vacuum is turned on the oil then boils and vaporizes. Like steam, the oil vapor is pushed through the bubbler caps and collection tray, and through the condenser where it is condensed back into liquid and collected in the holding tank. The variables of feed stock, temperature, process rate, and vacuum determine the grade of the final product.