So I was reading up the onwers manual about doing an oil change on the 8100 Vortec engine and they state that never use 10W40 or 20W50 wheight oil. Does somebody know what the cause of this is? I know many manufactors don't recommend this kind of wheight due to in winter or in cold conditions the oil is to heavy. But this manual states the big no at all conditions. Will it hurt the engine? Has someone tried it and had bad luck? Im guessing right off now that due to it beeing a tall deck motor the engine oil takes too long to go up to the lifters and so and that due to this there is wear of the valvetrain and cam,.. am i correct?