[HoosierDaddy]

Maybe I can shed some light on this debate by answering a question nobody has asked,,,.. Single weight oil molecules tend to be round. The weight of the oil is determined by the size of the molecule. Multiweight oil (say 10w30) is made by starting with 10 weight oil ( small molecule= less lubricating cushion than 30w). In order for the oil to achieve a 30w viscosity at high temperatures they add VI or viscosity improvers. The molecule of a VI is long and round,,,like spaghetti. It can be 1000 times longer than a 10w oil molecule. At 30 degrees the VI rolls up into a ball allowing the oil to effectively be 10w viscosity. As the temperature increases the VI tends to uncurl and stiffen thus causing the oil to thicken. The compromise here is that VI's by themselves have very little lubricating qualitys. Additionally they tend to shear under high pressure loads leading to a break-down of the oils viscosity over time. The larger the viscosity spread ie. 10w40 vs 10w30 the more VI is needed yielding an end product more susceptable to shear loads.
If I remember correctly Detroit Diesel specified single weight oils because of higher shear loading that their engine design placed on engine oil. Other engine designs specify a multi-weight oil partly because it "washes" over internal engine components faster thus carrying heat away faster.
IMHO a 10w30 or 10w40 used in place 0f a 30w for a short time under moderate conditions should not lead to immediate engine failure.
I am NOT a lubrication engineer and would yield to anyone on this board who is.!! This info is only from my own research.