Look what Redline says about their 15w-50

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Nov 16, 2002
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).
Nothing new here but, thicker then a 40wt is NOT recommended for most daily drivers. Why? For one it's a synthetic so the film strength of a 10w-30 synthetic is far greater then that of a petro 10w-30/40. Another thing is more heat is created due to lack of flow and you also end up with less HP/MPG as well. And last but not least, I'm an expert and know everything so sit down and shut up. [Big Grin] Just kidding. [ April 20, 2004, 08:08 PM: Message edited by: buster ]
my take on their viscosity recommendations is that they're looking at operational oil temps. for instance, if they feel that a viscosity of 15 Cst is preferable, they will spec an oil that is roughly 15 Cst at the temps you see. which might be about the case with their 15w50 running 240 degree oil temps, whereas below 225 a 10w40 would be closer. just a thought. from studying the oil pressure gauge, i can see how i'd want to run a 40w or 50w oil when pounding on the racetrack in summer (where oil temps can go to 300), but a 20w or 30w if highway cruising in winter (when the oil temps barely hit 200). -michael
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