The cold starting viscosity (ie at 40C) should be the same whether it's a 5W20, 5W30 or a 5W40 same thing when comparing a 10W30 to a 10W40 Red Line Synthetic Motor Oil. However on Red Line's spec sheet it lists the Vis at 40 C as 55 for the 5W20, 67 for the 5W30 and 94 for the 5W40? Similarly the 10W30 is 70 and the 10W40 it's 98?
If the startup Vis is technically the same (or very close to) for a 5W20, 5W30 or 5W40 then why is there such a big range in Vis at 40 C between the Red line 5W20 and the 5W40... same thing when comparing the 10W30 and the 10W40? The above is probably true for the other brands as well - why? I thought a 5W was a 5W at startup whether the second number was a 20 or 40?
If the startup Vis is technically the same (or very close to) for a 5W20, 5W30 or 5W40 then why is there such a big range in Vis at 40 C between the Red line 5W20 and the 5W40... same thing when comparing the 10W30 and the 10W40? The above is probably true for the other brands as well - why? I thought a 5W was a 5W at startup whether the second number was a 20 or 40?
