First, about viscosity: a guy at a motor oil company told me some years back that a 10W30 motor oil would be, at a “cool” temperature, behaving like a 10W oil, viscosity wise, but that the cool 10W would still be more viscous than a hot 30W. Does that sound right?

Second, why I ask: I have a ‘91 Corolla with 230k miles on it that puts out a nice puff of blue-white smoke when I start it. Friends who ought to know have told me that this is likely due to worn valve guides or valve seals. 4-AFE or 4-AGE engine; I don’t remember which. Some folks have said that running a straight-weight 30W could help that, or even mixing in a quart of 50W. Where I live, extreme Arctic blast is the mid 30s Fahrenheit.

What are your thoughts?