Obviously all fluids have viscosity so albeit an arbitrary way of labeling viscosity the system that we use for motor oil can only go so far without going negative or being adjusted accordingly. We are already down to 0w oil but that is obviously above zero cP so where does it go from here? Surley lower viscosities of oil are obtainable. Is SAE going to have to redefine its system or will we simply be talking of the newest -20w--5 and thinking people were insane for ever using 0w-20?