30 weight will "sludge up". That's funny.

The only downside of using 10/30 is that it won't flow as easily on startup in weather colder than 0F.

That's why in the winter it recommended to use 5/20 or 5/30.

And there will be almost zero gas milage differences with 5/20 and 5/30. A half mile per gallon max. under any conditions.

The way engines sludge is not changing out the oil frequently and/or owning sludge monster engines from Toyota, and Dodge Intrepids.

Use 5/20 MC or 5/30 MC with confidence under any driving conditions.