FYI, Gunk Motor Flush and K&W Motor Flush are composed of 60% - 100% diesel fuel per the MSDSs, so there's plenty of precedent for using diesel "safely" in crankcases for cleaning purposes. But that's as a temporary oil ADDITIVE, rather than as a 100% lube itself! Diesel is also used in at least a few "fuel system/injector cleaners," including CRC Fuel Injector Cleaner (>95% diesel), so it's also "safe" to add to gasoline tanks, but perhaps at 12 oz at a time. I'm not judging the effectiveness of either application, but simply mentioning that diesel in limited quantities is "safe" for both crankcases and gas tanks. (There's no absolute safety when using such chemicals...hence the quotes.)