Someone in another thread is "debating" whether 5W30 is thin oil with thickeners; or a thick oil with thinners...? I was always under the assumption that most oils start off as a rather thin base oil, using viscosity modifiers to get the desired operating temp. viscosity. ...And I was always told to use the narrowest viscosity spread possible, and use straight weights if conditions permit ... to high a spread (IE 5W50, 10W60) and the modifiers would break, or shear down, causing the oil to thin out, oxidize and leave deposits. Is this always the case; what about synthetics; how do the 0Wxx's work; why does a 15W50 M1 seem more stable than a 5W30...is a 15W40 the most stable oil, or maybe the 5W20's...? Comments...?