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I think SAE 50 is straight 50 weight no matter the temperature. 20W50 will change viscosity with temperature so cold starts will not be so hard on seals due to thick oil and high pressure. The downside is that the viscosity modifiers that allow that are not as durable so the oil will get thin and need changing more quickly. This is part of Castrol GTX's problem.
If you are racing this engine or live in a warm climate you might be able to get away with straight 50 weight if you are careful on cold starts. It will hold up to high temperatures better and will have lots of nice Zinc and Phosphorous that catalytic converters would choke on.
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1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
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