What are your opinions on those oils for high mileage enginges? Are they really any better for older vehicles than a good synthetic? I'm thinking of changing to Valvoline Maxlife Synthetic, or maybe mobil 1. My engine seems pretty dirty, so I'm looking for something that will clean it out well.

Also, a friend told me that synthetics alter the metal in the engine so that you can't machine it (to soft) for a rebuild. I know it is probably more a question of the type of alloy in the block and the additives in the oil, but does anyone know anything about that?

I've heard of changing weights for a high mileage engine, maybe to like 20w-50. What are the benefits and drawbacks?

Finally, what oil should I be using in a climate that ranges -10 to 35 degrees (yes that's Fahrenheit. Daytime temps. Not windchill. It's been down to -40 here at night) 5W-30? Probably not the 20w-50. Ha!

Thanks,
Newton