Found this on the Redline site, which is what I would also personally concur to with Amsoil synthetics.
quote:I feel that Oil temp has a big impact on determining what viscosity you chose. I would also say that most cars in America rarely see oil temps over 225°F unless they are pullling heavy loads, or else they have some serious cooling problems. Even at 100° a mechaniclly sound engine should be able to keep the water temps below 200°, which I would gather would have oil temps at 212 or below. Sustain high speed driving over 110 miles per hour would significnatly raise oil temps, but where can you do this at, except on a track. Also I would also say if oil temps were to get to 225°, as long as it's 15 minutes or less, you should have no problems, especially if your using a high quality syn. like mentioned above. Now my question is why don't they put oil temp guages on cars???
Red Line 15W50 & 20W50: The ultimate high-temperature protection in Red Line engine oils recommended for street use. Good for engines that regularly run very high oil temperatures. Best for engines that run large clearances such as air-cooled engines or large-displacement, all-out racing engines that see occasional street use. Provides 25% more viscosity in bearings than petroleum 20W-50s. Not recommended for use in cold climates where temperatures are at or below 10°F or -12°C. Not recommended for street use in production engines that see sustained oil temperatures below 225°F. Those engines should use Red Line 10W-30 or 10W-40.