[QUOTE="jim beam"] here's what i don't understand. i once saw a demo for some vi improvers, and it was fascinating to see viscosity /decrease/ with temperature and vice versa. i understand what you say about base stocks with naturally high viscosity indexes, i.e. ones that don't change much with temperature, but as i understand it, most bases still thin with increasing temp until the vi improvers are added, albeit some more than others.[/QUOTE] AFAIK, the only thing that VI improver does is reduce the temperature induced thinning - i.e. it's impossible to increase viscosity as temps increase. The 5W-30 (etc) just represents where the viscosity is at 0°C (5W) and at 100°C (30). [QUOTE] so, here's my question; why /wouldn't/ i want a lube with a low cool temp viscosity and higher hot temp one? something that maintains very similar viscosity vs. temperature is going to cost me more in gas, particularly in cold weather, and is not necessarily going to offer the same protection, right?[/QUOTE] Do you mean the number (5W/10W/20W/30/40/50) or do you mean the absolute viscosity. Like I said, it's impossible to increase the absolute viscosity as the temperature increases. The SAE's XW-YY scale is nothing more than an abstraction relative to "straight weight" oils. A 5W-30 oil doesn't actually increase in viscosity with temperature.
