I have asked before, but who makes Cat Oil? Cat does not refine oil, so who actually makes it? I guess being in the Lube business I would have to say that at least on some schedule, check and change oil--the stuff does get dirty and break down. Metal particles and other trash make their way into the crank case and clean oil is better than grit and grime. No combustion so no deterioration from fuel, but I would at least change oil that looks black and dirty. Just my opinion, but I do think it is sound regardless of the type oil that you use. The words: dirt, moisture, chemicals, filth, muck, sludge, and cylinder wear come to mind. Good things to avoid or at least minimize!