Originally posted by Drumbo: But, since the Mobil 1 5w-50 is a multi-vis, that should cover you under just about any condition -- assuming you don't spend too much time tooling around the Outback in the summer, where that 50 weight might kick in.
The highest it gets where I live is mid 40 degrees Celcius, and then only rarely. High 30's in summer is common. In winter 0 degrees first thing in the morning sometimes occurs.
The point with the Duratec though is it is designed to run on 5W30 for all of these temperatures. A 5W50 oil will be thicker than the engine is designed to run, then you run the risk of the oil not circulating quick enough throughout the engine.
I think that would be more of a concern with a straight-50 weight. A multi-vis behaves as its name implies: its viscosity changes with the engine's operating temperature; it doesn't necessarily attain the properties of a 50-weight every time out.
But this is general information, only. Whether your driving style, ambient conditions and the characterictics of your engine would cause the 5w-50 to attain a viscosity higher than 30, I couldn't say. Maybe somebody else will, uh, weigh-in on this point.
