re w w or w in article foo brad thone c bt wuvmd writes well there is a difference i don t happen to have my sae manual handy but oil viscosity in general decreases with temperature the sae numbers are based on a typical curve that oils used to all have running from say the viscosity of a room temperature weight at c down to say that of a room temperature weight at c for a typical weight oil oils that are designed for operation in normal temperatures just have a weight specification oils that are designed for operation in exceedingly cold temperatures have a w tacked on the end so in winter in a cold place you d stick w in your car in the winter and in it in the summer to approximate the appropriate viscosity throughout the year modern multi viscosity oils change viscosity much less with temperature as a result their viscosity graphs cross over several curves a multi vis specification pegs the curve at two temperatures a normal operating temperature and a cold one though i can t remember the numbers in any event the weights do indicate a significant difference remember that your engine is temperature regulated by the thermostat and radiator or air fins most of the time unless you overheat it or something any weight of oil is better than no oil or than very old carbonized oil thin oil won t in general lubricate as well at temperature thicker oil will like a w will lubricate better at temperature but not as well during startup when most engine wear occurs if you re planning on making long drives the w is probably fine esp in the summer in your w car but if you re making short drives stick to the w don t drink soap dilute dilute ok
