[sperry]

I guess I should have read your other post more carefully. Oil weight is written like "5W-30", which I learned to think of like "5 Winter (the cold viscosity) dash 30 (the hot viscosity)".

But the article is still correct in that 20W oil is going to be bad for a motor that expects 5W or 0W when it's cold. Like they said, racers tend to use heavier oil to fight the viscosity breakdown that happens when you over temp the oil. Since that's not happening at autocross, there's no gain for using the thicker oil.

I think your logic for the 5W-40 makes sense. 5W is going to be close to (exactly?) the factory recommended cold weight, while the 40 should afford a little extra protection on hot days when running hard, at a small expense of fuel mileage and horsepower. I just wouldn't bother with using non-factory recommended oil weights unless you are in fact running the car hard in the hot. 5W-40 probably won't make a huge difference, but running 15W-50 or something in a street car that never really gets super hot oil temps is probably going to accelerate wear on the motor over the 0W-30/5W-30 factory spec oil.